The frequency and general prevalence of a
phenomenon which is probably dependent on the raised temperature of the
deepest molten strata explain its independence of the nature of the mineral
masses in which it manifests itself. Earthquakes have even been felt in the
loose alluvial strata of Holland, as in the neighborhood of Middleburg and
vliessingen on the 23d of February, 1828. Granite and mica slate are shaken
as well as limestone and sandstone, or as trachyte and amygdaloid. It is
not, therefore, the chemical nature of the constituents, but rather the
mechanical structure of the rocks, which modifies the propagation of the
motion, the wave of commotion. Where this wave proceeds along a coast, or
at the foot and in the direction of a mountain chain, interruptions at
certain points have sometimes been remarked, which manifested themselves
during the course of many centuries. The undulation advances in the depths
below, but is never felt at the same points on the surface. The Peruvians*
say of these unmoved upper strata that "they form a bridge."
[footnote] *In Spanish they say, 'rocas que hacen puente'. With this
phenomenon of non-propagation through superior strata is connected the
remarkable fact that in the beginning of this century shocks were felt in
the deep silver mines at Marienberg, in the Saxony mining district, while
not the slightest trace was perceptible at the surface.
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