Where we meet with exceptions to this law, to which I long since drew
attention, they must depend upon the circumstance that the connections
between the volcanic foci and the crater of eruption can not be considered
as equaly permanent in the case of all volcanoes. The channel of
communication may be closed for a time in the case of the lower ones, so
that they less frequently come to a state of eruption, although they do not,
on that account, approach more nearly to their final extinction.
These relations between the absolute height and the frequency of volcanic
eruptions, as far as they are externally perceptible, are intimately
connected with the consideration of the local conditions under which lava
currents are erupted. Eruptions from the crater are very unusual in many
mountains, generally occurring from lateral fissures (as was observed in the
case of Aetna, in the sixteenth century, by the celebrated historian Bembo,
when a youth*), whenever the sides
p 230
of the upheaved mountain were least able, from their configuration and
position, to offer any resistance.
[footnote] *Petri Bembi Opuscula ('Aetna Dialogus'), Basil, 1556, p. 63:
"Quicquid in Aetnae matris utero coulescit, nunquam exit ex cratere
superiore, quod vel eo inscondere gravis materia non queat, vel, quia
inferius alia spiramenta sunt, non fit opus.
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