Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

March, 1835. earthquake waves. 377 opinion, we have scarcely beheld since leaving England, any- other sight so deeply interesting. In almost every severe earthquake which has been de- scribed, the neighbouring waters of the sea are said to have been greatly agitated. The disturbance seems generally, as in the case of Concepcion, to have been of two kinds : first, at the instant of the shock, the water swells high up on the beach, with a gentle motion, and then as quietly retreats ; secondly, some little time afterwards, the whole body of the sea retires from the coast, and then returns in great waves of overwhelming force. The first and less regular movement seems to be an immediate consequence of the earthquake differently affecting a fluid and a solid, so that their respective levels are slightly deranged. But the second case is a far more important phenomenon, and at first ap- pears of less easy explanation. In reading accounts of earthquakes, and especially of those on the west coast of America, as collated from various authors by Sir W. Parish,* it is certain that the first great movement of the waters has been that of retiring. Several hypothesesf have been invented to explain this fact. Some have sup- posed it owing to a vertical oscillation in the land, the water retaining its level : but this can hardly happen, even on a moderately shoal coast; for the water near the land must partake of the motion of the bottom. Moreover, as Mr. LyeU has urged, a change of level in the land will not account for movements in the sea, of a similar nature, affecting islands distant from the line of uplifted coast. This occurred at Madeira during the famous Lisbon earthquake. Juan Fernandez also ofifers a parallel instance ; for the sea was disturbed there much in the same manner as on the coast of Chile. The whole phenomenon, it appears to me, is due to a com- mon undulation in the water, proceeding from a line or point of * Sir W. Parish had the kindness to lend me the original manuscript, which was read before the Geological Society, March 3th, 1833. f Lyell's Geology, book ii., ch. xvi.