FORMATION OF CORAL REEFS AND ISLANDS. 237 
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 
greatest force would be felt not far from the line of high tide, 
or between that line and three feet above it; moreover, the 
rise of the waters to half or two-thirds tide affords a protection 
against the breaker to whatever is below this level. In re- 
gions 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 influence 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. 
Besides a line of greatest wave-action, we also distinguish 
a height of feeble action, —so feeble that the rock remains 
unremoved along and below a nearly horizontal plane which 
is often three hundred to four hundred yards in width. The 
height, as is evident from the facts stated, is some distance 
above low-tide level. The lower waters of the tide, besides 
being protective, as above explained, are accumulative in their 
ordinary action, when the material exposed to them is moy- 
able; they transport shoreward, while the upper waters are 
eroding, and preparing material to be carried off. The height 
at which these two operations balance each other will be the 
height, therefore, of the line of least degradation. Moreover, 
it should vary with the height of the tides. This height, on 
the eastern shores of Australia, is three feet above ordinary 
low tide, and at New Zealand about five feet. With regard 
