The waves of an earthquake have been represented in their progress,
and their propagation, through rocks of different density and
elasticity; and the causes of the rapidity of propagation, and its
diminution by the refraction, reflection, and interference of the
oscillations have been mathematically investigated. Air, water, and
earth waves follow the same laws which are recognised by the theory of
motion, at all events in space; but the earth-waves are accompanied
in their destructive action by discharges of elastic vapours, and
of gases, and mixtures of pyroxene crystals, carbon, and infusorial
animalcules with silicious shields. The more terrific effects are,
however, when the earth-waves are accompanied by cleavage; and, as in
the earthquake of Riobamba, when fissures alternately opened and closed
again, so that men saved themselves by extending both arms, in order to
prevent their sinking.
As a remarkable example of the closing of a fissure, Humboldt mentions
that, during the celebrated earthquake in 1851, in the Neapolitan
province of Basilicata, a hen was found caught by both feet in the
street-pavement of Barile, near Melfi.
Mr. Hopkins has very correctly shown theoretically that the fissures
produced by earthquakes are very instructive as regards the formation
of veins and the phenomenon of dislocation, the more recent vein
displacing the older formation.