Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/428

Rh, which is always, intermediately transverse both to the wave-path and to the slope of the subjacent rock, and very commonly to the slope of the earth's surface itself, which more or less corresponds to that of the rock beneath.

That side of the lips of the fissure, next the descent of the sloping surface is always found lower than the other as due to the original slope of surface.

Upon level ground a fissure in the earth has not been found, unless the earth had one, unsupported or relatively unsupported, side, as on the Campostrina Road and near Polla.

These combined facts, prove beyond question, that earth fissures are not produced by the direct action of the wave of shock, but that they are merely a secondary effect—cases of small and incipient landslips, produced by the shaking downwards of the mass, of earth reposing loosely upon the sloped surface of subjacent rock; or comparatively unsustained in any way at one side, as at Campostrina, where, the earth of the road, resting there upon a sloped surface too, was only upheld by a revetment wall.

It is untrue to nature, to state that, "the wavy nature of the shocks, occasions such a stretching of portions of the ground, as sometimes to split it asunder. Hence, amongst the consequences resulting from it, in the earth itself, fissures are the most frequent." (Daubeny, 'Volcanoes,' p. 629, edit, of 1848.) No earthquake wave, with an amplitude not exceeding 6 or 7 inches at the most, could, by possibility, produce any such stretching; nor were this amplitude increased, in the ratio that the intensity of South American earthquakes bears to that of this Neapolitan one,