160 DYNAMICAL GEOLOGY. 



Suppose, then,, a concussion, from any cause, beneath 

 the sea-bed. There would be formed, as before, about 

 the focus, a series of spherical earth-waves, which, by en- 

 largement, would emerge on the surface of the sea-bed 

 as circular surface-waves. These, spreading beneath the 

 sea, would reach the nearest shore, and produce their de- 

 structive effects there. Some time afterward, perhaps 

 a half -hour or more, there comes rolling in 011 shore a 

 prodigious water wave, or perhaps a series of water waves, 

 thirty to sixty feet high, deluging the whole shore region, 

 and completing the destruction commenced by the earth- 

 wave. 



The Great Sea Wave. This very destructive accom- 

 paniment of earthquakes occurring beneath offshore sea- 

 beds may be explained as follows : The bed of the sea 

 at the epicentrum is lifted up perhaps several times. 

 This lifts the whole sea water above, so that the surface 

 is raised into a water mound. This mound immediately 

 sinks as much below the sea level as it was before raised 

 above it, and thus gives origin to a circular water wave 

 (or series of such waves) which spreads exactly like any 

 other water wave, growing lower as its spreads, until it 

 breaks on the nearest shore. Out at sea such great low 

 waves would pass under a ship unobserved, heaving it 

 slowly up and letting it down again. But when they 

 approach shore, on account of their great size, often fifty 

 feet high and one hundred to two hundred miles across 

 the base, they rush forward as a tide fifty feet high and' 

 devastate the whole coast within their reach. They are, 

 therefore, sometimes called tidal waves, although they 

 have nothing to do with tides. Though originating at the 

 same place, the great sea wave moves much less rapidly 

 than the earth-wave, and therefore reaches the shore later. 



Examples of the Great Sea Wave. 1. In 1755 a 

 terrible earthquake destroyed Lisbon, and, it is said, forty 

 thousand people. The focus of this earthquake was 



