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. Lyell 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 offers 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 5tb, 1835. 

 f Lyell's Geology, book ii., ch, xvi. 



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