Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/432

Rh fractures or fissures, fresh formed, directly through hard rock, but they were not due to either bending or stretching of the strata by the roll of the wave itself in transit; they were still mere secondary effects, due to the push at the instant, produced by the wave at the free-lying surface of the hill side, acting upon the enormous mass of clays and shales, &c., behind. The rock there was rent, just as the wall of a water reservoir above ground, might be rent or broken transversely by the pressure from within.

Falls of rock were observed, and upon a very great scale, at Campostrina, Vallone del Raccio, Padula Valley, La Scorza, La Sala, Arena Bianca, and Monticchio, and were recorded in the 'Giornale Reale' as having taken place also at Vesalo, near Laurino, and at Pietra Pertosa. These were all, instances of transverse fracture and separation, at rock joints already existing, and due to inertia.

The occurrence of rock falls, and the scale upon which they are presented, depends more upon the energy of the earthquake, than in the cases of earthslips, but this is also, to a great degree, determined by the physical features of the country. If the rock be, ill-coherent, shattery material, like the cretaceous limestones of the Apennines, larger and more numerous dislodgments will occur, than in a country formed all of bare, rigid, crystalline rocks, and whose surface has been perhaps swept clean, by denudation, from all loose material, upon the elevated points; so also if it be one of steep hillsides, and deep valleys, with slopes often encumbered with boulders, &c., as in the Apennines, great and frequent dislodgments will be found. More numerous falls will take place from rock masses, whose bedding is