232 CORALS AND COMAL ISLANDS. 



conical crown. The water, in these cases, has worn away the 

 cliffs, leaving the basement untouched. 



A surging wave, as it comes upon a coast, gradually rears it- 

 self on the shallowing shores; finally, the waters at top, through 

 their greater velocity, plunge with violence upon the barrier 

 before it. The force of the ocean's surge is therefore mostly 

 confined to the summit waters, which add weight to superior ve- 

 locity, and drive violently upon whatever obstacle is presented. 

 The lower waters of the surge advance steadily but more 

 slowly, owing to the retarding friction of the bottom; the 

 motion they have is directly forward, and thus they act with 

 little mechanical advantage ; moreover, they gradually swell 

 over the shores, and receive, in part, the force of the upper 

 Waters. The wave, after breaking, sweeps up the shore till it 

 gradually dies away. Degradation from this source is conse- 

 quently most active where the upper or plunging portion of 

 the breaker strikes. 



But, further, we observe that at low-tide the sea is compara- 

 tively quiet ; it is during the influx and efflux that the surges 

 are heaviest. The action commences after the rise, is strongest 

 from half to three-fourths tide, and then diminishes again near 

 high tide. Moreover, the plunging part of the wave is raised 

 considerably above the general level of the water. From 

 these considerations, it is apparent that the line of greatest 

 wave-action must be above low- water level. Let us suppose a 

 tide of three feet, in which the action would probably be 

 strongest when the tide had risen two feet out of the three ; 

 and let the height of the advancing surge be four feet : — the 

 wave, at the time of striking, would stand, with its summit, 

 three feet above high-tide level ; and from this height would 

 plunge obliquely downward against the rock, or any obstacle 

 before it. It is obvious that, under such circumstances, the 



