important agent in volcanic action. A geyser, in fact,
may be designated as a water volcano, since it throws up water only. It
comprises a cone or mound, usually only a few feet high. In the middle
of this is a crater-like opening with a passage leading down into the
earth. As in the case of the volcano, the geyser cone is built up by its
own action. In the boiling water which is ejected there is dissolved a
certain amount of silica. As the water falls and cools this mineral is
deposited, gradually building up a cup-like elevation. The basin of the
geyser is generally full of clear water, with a little steam rising
from its surface; but at intervals an eruption takes place, sometimes at
regular periods, but more often at irregular intervals.

Among the largest and best known geysers in the world are those of
Iceland, chief among them being the Great Geyser. Silica is the
mineral with which the waters of this fountain are impregnated, and
the substance which they deposit, as they slowly evaporate, is named
siliceous sinter. Of this material is composed the mound, six or seven
feet high, on which the spring is situated. On the top of the mound is
a large oval basin, about three feet in depth, measuring in its larger
diameter about fifty-six, and in its shorter about forty-six feet. The
centre of this basin is occupied by a circular well about ten feet in
diameter, and between seventy and eighty feet deep.

Out of the central well springs a jet of boiling water, at intervals of
six or seven hours. When the fountain is at rest, both the basin and the
well appear quite empty, and no steam is seen. But on the approach of
the moment for action, the water rises in the well, till it flows over
into the basin. Then loud subterranean explosions are heard, and the
ground all round is violently shaken.

Instantly, and with immense force, a steaming jet of boiling water, of
the full width of the well, springs up and ascends to a great height
in the air. The top of this large column of water is enveloped in vast
clouds of steam, which diffuse themselves through the air, rendering it
misty. These jets succeed each other with great rapidity to the number
of sixteen or eighteen, the period of action of the fountain being about
five minutes. The last of the jets generally ascends to the greatest
height, usually to about 100, but sometimes to 150 feet; on one occasion
it rose to the great height of 212 feet. Having ejected this great
col