25.
The intelligent geographer of Amasea, to whom we are indebted for the notice
of this circumstance, further remarks: "Since the craters of Aetna have
been opened, which yield a passage to the escape of fire, and since burning
masses and water have been ejected, the country near the sea-shore has not
been so much shaken as at the time previous to the separation of Sicily from
Lower Italy, when all communications with the external surface were closed."
We thus recognize in earthquakes the existence of a volcanic force, which,
although every where manifested, and as generally diffused as the internal
heat of our planet, attains but rarely, and then only at separate points,
sufficient intensity to exhibit the phenomenon of eruptions. The formation
of veins, that is to say, the filling up of fissures with crystalline masses
bursting forth from the interior (as basalt, melaphyre, and greenstone),
gradually disturbs the free intercommunication of elastic vapors. This
tension acts in three different ways, either in causing disruptions, or
sudden and retroversed elevations, or, finally, as was first observed in a
great part of Sweden, in producing changes in the relative level of the sea
and land, which, although continuous, are only appreciable at intervals of
long period.
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