It is not by their ejected lavas alone that the volcanoes of
Iceland produce their destructive effects. Disastrous consequences
have frequently resulted from the sudden melting of their snows and
glaciers, on which the volcanic fires operate far more rapidly than
does the heat of the sun. It is chiefly by the vast quantities of
earth, sand, stones, and broken fragments of rock, which they hurry
along with them in their wild career, that the waters, so suddenly
freed, produce the greatest amount of damage. During an eruption of
Katlugaia, one of the southern Icelandic volcanoes, in 1756, the
mass of material thus carried down by the melted snows and glaciers
was so great, that, advancing several leagues into the sea, it
formed three parallel promontories, which rose above the sea-level,
where there had formerly been a depth of forty fathoms of water.
Vast ravines were, at the same time, scooped out of the sides of
the mountain by the erosion of the waters. Another eruption of this
volcano in 1860 produced similar results.

Still more interesting than the volcanic mountains of Iceland are
its Geysers, or intermittent springs of boiling water. The chief of
these is the Great Geyser. A jet rises to a vast height, and is
accompanied by much steam. Indeed, it is quite at the boiling-
point.

The little mound, from the top of which the jet appears to rise, is
composed of a substance named siliceous sinter, and is a deposit
from the water of the fountain. At the top of this mound, which is
between six and seven feet in height, there is an oval basin,
measuring about fifty-six feet in one direction, and about forty-
six in the other; its average depth is about three feet. In the