EARTEQ UAKE-PEENOMENA. 



519 



disastrous in the tertiary and blue clay on which the ruined portion of 

 the city stood. 



The sea-wave put in motion by this earthquake exceeded in volume 

 all others of which we have a record, except the one already noticed, 

 which traversed the Pacific Ocean in 1868. It was observed, during 

 this convulsion, that the sea retired from the shore before the great 

 wave rolled in. 



Fig. 5. 



Curved Strata, as seen m the Swiss Juba, 



It was Darwin who first suggested that waves first draw the 

 waters from the shore on which they are advancing to break. He 

 calls attention to the familiar fact that waves thrown up by the pad- 

 dles of a steamer, as they approach the shore, are alway preceded by a 

 receding of the water. An under-draught seems to first suck the wa- 

 ter back, and such actually is the fact. Now, in the sea-wave raised 

 by the earthquake, what takes place ? We have remarked that an 



Fig. 6. 



f d c h a 



Strata broken and displaced. 



earthquake is a vibration of the earth's elastic crust, and moves with 

 tremendous velocity. When it occurs beneath the sea, or when the 

 undulations reach the surface beneath the sea, the motion is com- 

 municated to the water, which it elevates in a wave. Simultaneously 

 with this lifting of the water, an under-draught toward that point takes 

 place. Were it not so, the elevation of the wave could not be sus- 

 tained. Directly the great wave moves from the area of disturbance 



