covered by volcanic sand showered down upon it
before the descent of the lava. A dense stratum of this fine dust mixed
with scoriae is well known to be an extremely bad conductor of heat; and
the shepherds in the higher regions of Etna are accustomed to provide
water for their flocks during summer, by strewing a layer of volcanic
sand a few inches thick over the snow, which effectually prevents the
heat of the sun from penetrating.

Suppose the mass of snow to have been preserved from liquefaction until
the lower part of the lava had consolidated, we may then readily
conceive that a glacier thus protected, at the height of ten thousand
feet above the level of the sea, would endure as long as the snows of
Mont Blanc, unless melted by volcanic heat from below. When I visited
the great crater in the beginning of winter (December 1st, 1828), I
found the crevices in the interior incrusted with thick ice, and in some
cases hot vapors were actually streaming out between masses of ice and
the rugged and steep walls of the crater.[572]

After the discovery of Signor Gemmellaro, it would not be surprising to
find in the cones of the Icelandic volcanoes, which are covered for the
most part with perpetual snow, repeated alternations of lava-streams and
glaciers. We have, indeed, Lieutenant Kendall's authority for the fact
that Deception Island, in New South Shetland, lat. 62 degrees. 55
minutes S., is principally composed of alternate layers of volcanic
ashes and ice.[573]

_Origin of the Val del Bove._--It is recorded, as will be stated in the
history of earthquakes (ch. 29), that in the year 1772 a great
subsidence took place on Papandayang, the largest volcano in the island
of Java; an extent of ground _fifteen miles in length_, _and six in
breadth_, covered by no less than forty villages, was engulphed, and the
cone lost 4000 feet of its height. In like manner the summit of
Carguairazo, one of the loftiest of the Andes of Quito, fell in on the
19th July, 1698; and another mountain of still greater altitude in the
same chain, called Capac Urcu, a short time before the conquest of
America by the Spaniards.

It will also be seen in the next chapter that, so late as the year
1822, during a violent earthquake and volcanic eruption in Java, one
side of the mountain called Galongoon, which was covered by a dense
forest, became an enormous gulf in the form of a semicircle. The new
cavity was about midway between the summit and the