This is a very important matter in all mountainous districts,
particularly where the rainfall is light.

The Yellowstone National Park is situated upon the continental
divide in northwestern Wyoming. It is largely a plateau, with an
elevation of seven thousand to eight thousand feet above the level
of the sea. The surface of the plateau is covered with forests,
meadows, and lakes; but the region is particularly remarkable for
the geysers and hot springs, and the Grand Canon and falls of the
Yellowstone River.

Springs dot the surface of many parts of the park. The hot water
is continually bringing mineral substances, the chief of which is
silica, from the depths of the earth and depositing them about the
orifices of the springs. In this manner wonderful basins, terraces,
and cones have been built up, while the rocks have been either
reddened or bleached out and softened into a form of clay.

The park region must have been for a long period the seat of volcanic
action, for nearly all the rocks are cooled lavas. While the heat
has disappeared from the surface, it must still be very great below,
if we may judge by the quantities of hot water continually issuing
from the springs.

In many a subterranean cavern steam accumulates until its pressure
becomes too great for the column of water occupying the channel
that leads to the surface; then the water is suddenly and forcibly
expelled, giving rise to a geyser eruption. When the pressure of
the steam has become exhausted, the water sinks back into the earth,
leaving the basin of the geyser nearly or quite empty until the steam
has again collected. Each geyser has its own period of eruption
and is generally very regular. One little geyser, known as the
Economic, because it throws out but little water, spouts regularly
about every five minutes. Other geysers are active at intervals of
several hours, while some take several years to get ready for a
new eruption and then spout whole rivers of boiling water. In the
Upper Geyser Basin the effect is very impressive, particularly upon
a cool morning. The clouds of steam and the throbbing or roaring
geysers lend to the region a weird and unearthly aspect.

The Yellowstone Lake is a large body of water situated almost upon
the continental divide. Before the canon, or Great Falls, or even
the Yellowstone River itself existed, the lake stood about one
hundred and fifty feet higher than at present, and its water emptied
into the Pa