Russell] SPRINGS. 151 
At the left of the diagram an attempt has been made to indicate the 
presence of a mountain valley deeply filled with alluvium, beyond 
which rises a deeply sculptured, bordering- mountain spur. The gravel 
in the valley extends beneath the plain, perhaps merging with lake 
beds, and forms a porous layer which is underlain by an imper- 
vious bed and covered by a lava sheet. This is a special case, to be 
sure, but the beds beneath the plain may be of any nature, the essen- 
tial feature being that one of them is so situated as to become water 
charged and to slope toward a canyon which is cut below its base. The 
bed above the pervious layer in the case of the Snake River Plain is 
composed of fissured lava, and permits all the water falling on it to have 
ready access to the porous bed. beneath. In fact the fissured basalt at 
times furnishes the pervious bed, being underlain by a water-tight 
stratum. Water coming to the exposed portion of the pervious bed 
in the mountains descends into it, as does also the water supplied to 
the surface of the plain, as is indicated by arrows. Where the pervi- 
ous bed appears in the side of a carbon, springs emerge. 
The lost rivers of southern Idaho contribute to the support of the 
springs in Snake River Canyon, but this source of supply is augmented 
by the precipitation on the adjacent plain. The flow is constant, o wing- 
to the retention of the waters in porous beds, being in part supplied 
by hillside springs in the mountains, but the flow is regulated more 
largely by the extent of the water-charged beds. In general, canyon 
springs are perennial and exhibit no seasonal changes in volume or 
temperature. 
FISSURE SPRINGS. 
If a pervious layer of the nature of those supplying canyon springs 
is not cut by erosion, or in some other way furnished with means of 
escape for its contained waters, it is evident that it may become water 
filled. The water at any point in the porous bed will then be subject 
to the pressure of water at higher levels, and be forced in any direc- 
tion in which an opening may occur. In nature such means of escape 
are sometimes provided by fissures. If a fissure leads from such a 
reservoir to the surface and the pressure on the water is sufficient, a 
fissure spring results. The conditions which lead to the occurrence of 
fissure springs are seldom, if ever, exposed so as to be studied in detail, 
and are presumably varied. In many instances it is probable that a 
porous stratum is absent, and the water finds its way through inter- 
secting fissures. However diverse the conditions, it is evident that the 
water which rises and overflows is under pressure, and that a reservoir 
is present. By this it is not meant that anything like a cavern neces- 
sarily or commonly occurs. Usually porous rocks play the role of a 
reservoir. Fissure springs, like all other springs, are dependent on 
precipitation for their water supply, but owing usually to the small 
