It is impossible, without an exact
representation of the configuration -- the normal type, as it were, of
fire-emitting mountains, to form a just idea of those phenomena which, owing
to fantastic descriptions and an undefined phraseology, have long been
comprised under the head of 'craters, cones of eruption', and 'volcanoes'.
The marginal ledges of craters vary much less than one would be led to
suppose. A comparison of Saussure's measurements with my own yields the
remarkable result, for instance, that in the course of forty-nine years
(from 1773 to 1822), the elevation of the northwestern margin of Mount
Vesuvius ('Rocca del Palo') may be considered to have remained unchanged.*
[footnote] *See the ground-work of my measurements compared with those of
Saussure and Lord Minto, in the 'Abhandlungen der Akademie der Wiss. zu
Berlin' for the years 1822 and 1823.
Volcanoes which, like the chain of the Andes, lift their summits high above
the boundaries of the region of perpetual snow, present peculiar phenomena.
The masses of snow, by their sudden fusion during eruptions, occasion not
only the most fearful inundations and torrents of water, in which smoking
scoriae are borne along on thick masses of ice, but they likewise exercise a
constant action, while the volcano is in a state of perfect repose, by
infiltration into the fissures of the trachytic rock.
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