236 CORALS AND CORAL ISLANDS. 
conical crown. The water, in these cases, has worn away the 
cliffs, leaving a broad horizontal basement above the level of 
low tide. 
According to Professor Verrill, the same feature is exhib- 
ited on a grand scale at the island of Anticosti im the Gulf 
of St. Lawrence, and “Old Hats” are among the forms 
produced. The cliff consists of limestone, and the “ shore- 
platform” is in many places over four hundred yards wide. 
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 wpper 
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 ~ 


