152 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
size of the conduits leading to the surface, in comparison to the volume 
of the reservoir, they are perennial and do not fluctuate in volume or 
temperature with seasonal changes. Like canyon springs, those of 
the type here considered are supplied from a distance and may exist in 
arid regions. They are frequently of common occurrence where broad 
sheets of stratified rock have been broken, and especially where faults 
occur. Owing to the depth from which their water frequently rises 
its temperature is often high. Springs of this type are common in the 
Great Basin, and occur also throughout the similarly disturbed region 
about it. In southern Idaho numerous examples are present, as is 
shown by the several warm and hot springs known. 
CAVERN SPRINGS. 
In limestone regions especially, water percolating through the 
rocks, on finding its way initially through fissures and joints, dissolves 
the rocks and opens out galleries. These galleries, such as Mammoth 
Cave in Kentucky, for example, may have lakes in them, or the water 
may flow freely; in either case a spring is formed where the water 
emerges in a neighboring valley. If a subterranean lake is present it 
may supply a conduit with water under pressure, and give origin to a 
fissure spring. With or without a lake present, springs in the sides of 
valleys may be found which simulate canyon springs. Caverns origi- 
nating in any manner may exert an influence on subterranean drain- 
age in the ways just suggested. 
Cave springs are not known in southern Idaho, and further discus- 
sion of the conditions on which they depend need not be indulged in 
at this time. 
Similar to cave springs are the streams which sometimes gush out 
of glaciers, but these have no connection with tin 1 subject in hand. 
TEMPERATURE. 
In all springs the temperature of the water is regulated by the tem- 
perature of the rocks from which it is derived. 
In this connection certain well-known facts may be restated. The 
seasonal variations in temperature in temperate regions extend down- 
ward into the earth to a depth of about 50 feet, and at that depth there 
is a horizon of no seasonal variation which has a temperature corre- 
sponding with that of the mean annual temperature at the surface. 
Below this stratum of no seasonal variation the temperature increases 
with increase in depth, at the rate in general of about one degree 
Fahrenheit for each 60 feet. 
Hillside springs, being of local origin and the waters supptying them 
not descending deeply, usually have about the mean annual tempera- 
ture of the region where they occur. At Boise this is approximately 
50° F. In summer such springs are colder and in winter warmer than 
