110 CORAL FORMATIONS. 
a horizontal surface; it was a direct result from the causes at work. 
The shore shelf stands about five feet above low water. A small island 
in this bay is well named the “ Old Hat,” the platform encircling it, 
as shown in the preceding figure, forming a broad brim to a rude 
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 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 presented. The dower 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 consequently most active 
where the upper or plunging portion of the breaker strikes. 
But, further, we observe that at low tide the sea is comparatively 
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 cir- 
cumstances, the greatest force would be felt, not far 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, as six feet, for example, 
the same height of wave would give nearly the same height to the 
line of wave-action, as compared with high tide level. Under the in- 
fluence of heavier waves, such as are common during storms, the line 
of wave-action would be at a still higher elevation, as may be readily 
estimated by the reader. 
