MOVEMENTS OF THE SEA-FLOOR 213 



varied. In fresh- water lakes and lesser seas the tidal wave 

 is not developed at all. In greater basins such as the Med- 

 iterranean the undulations are so slight that their influence 

 is scarcely perceptible ; but about the head of such oceanic 

 reentrants as the North Atlantic, where the wave rolling in 

 from the south is compressed between converging shores, a 

 vast amount of tidal work is done. 



The fourth group of actions effective on sea-shores is 

 derived from the movements of subsidence, elevation, and 

 sudden jarring which may take place on the sea-floor. Some 

 of the most important alterations of our harbors are effected 

 by the uprising or downsinking of the portions of the earth 

 near which they are situated. Movements of this sort, if 

 they take place over very extensive portions of the ocean- 

 floor, may, by displacing the water of all the oceans and 

 open seas, lead to changes in the depth of water in harborages 

 all over the world. Thus, if, as seems probable, an area 

 of several million square miles in the central district of the 

 Pacific Ocean is undergoing subsidence at the present time, 

 the influence of the movement, long continued and extensive, 

 may be felt in all marine ports. In general, however, local 

 accidents in the way of subsidence and elevation are so con- 

 siderable as to make these wide-spread oscillations of the 

 ocean-floor relatively unimportant. 



Besides the long-continued oscillations of level of shore 

 land and sea bottom, an effect is exercised upon the coast- 

 line through the sudden movements of the earth beneath the 

 seas, due to earthquake shocks. These seismic jarrings. 

 create an elevation of the water over the shaken surface, 

 which is in its nature a broad wave, having, it mav be, 

 an altitude of several feet, and which maN' roll across the 



