and, far more strikingly, in the bed of the Agri, in the plain of Viggiano, where the great extent of land, shaken and slipped down, into the bed of that torrential river, well illustrates the large scale upon which, these secondary effects may alter and modify, the local features of an earthquake country.
It is to be borne in mind, however, that the magnitude and the superficial extent of such effects, depend but very subordinately, upon the power, or energy of the earthquake, and are mainly dependent upon the conditions of unstable equilibrium presented by great masses of loose material through the configuration of the country. If there be plenty of huge river and other banks, formed of steep masses of clays, &c.; or of great masses of rock, resting upon wet and unctuous beds of shale, or of clays, or such-like, a very moderate shock, may cause prodigious alteration of surface in this way. Thus it was that the quay of Lisbon slipped with its supporting bank of blue clay, into deep water.
How small a shock, would have been sufficient to have determined the fall of the Rosberg, long before by gravity only, it took place, or would have anticipated and extended, the great landslip at Lyme Regis, as described by Mr. Roberts — how small a vibration would be sufficient, to cause vast masses, of the southern side of the Isle of Wight, to launch into the English Channel, or to fissure the steep slopes of the Hampstead and Highgate hills near London.
Direct fissuring of rock by the transit of the wave of shock, is more a physical possibility, than that of earth, yet it remains doubtful if such has ever taken place.
At Arena Bianca, I have recorded the particulars of
