On Coral Reefs and Islands. 335 



F 



causes at work. The shore shelf stands about five feet above 

 low water. A small island in this bay is well named the " Old 



« „ Qj^j, HAT," BAT OF ISLANDS. 



n. 



Hat," the platform encircling it, as shown in the above figure, 

 forming a broad brim to a rude conical crown. The water, in 



these cases, has worn away the cliffs, leaving the basement un- 

 touched. 



A surging wave, as it comes upon a coayt, gradually rears itself 

 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 surges is therefore mostly confined to 

 their summit waters, which add weight to superior velocity, and 

 drive violently upon whatever obstacle is preseiited. The lower 

 waters of the surge advance steadily but more slowly, owm^ to 

 the retarding friction of the bottom; the motion they have is 

 direcily forward, and thus they act with little mechanical advan- 

 fage ; moreover, they gradually swell over the shores, and receive, 

 1"^ part, the force of the upper waters. The wave, after break- 

 ing, sweeps up the shore till it gradually dies away. Degradation 

 from this source is consequently most active where the upper or 

 phinging 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 

 naif to three-fourths tide, and then diminishes again near high 

 tide. Moreover, the plunging part of the wave is raised consid- 

 erably above the general level of the water. From these con- 

 siderations, it is apparent that the line of greatest wave-action, 

 JJiust 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 w^ave, at the time of 

 striking, would stand, with its summit, three feet above high tide 

 level; and from this height would phinge obliquely downward 

 against the rock, or any obstacle before it. It is obvious, that 

 ^nder such circumstances, the greatest force would be felt, not 

 ^^r from the line of high tide, or between that line and three feet 

 above it. In regions where the tide is higher than just supposed, 

 ^ six feet for example, the same height of wave would give 



