FORMATION OF CORAL BEEFS AND ISLANDS. 233 



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 the greatest wave-action, we may also dis- 

 tinguish a height where this action is entirely null ; and it is 

 evident, from facts already stated, that the point will be found 

 somewhat above low- tide level. The lower waters of the surge, 

 instead of causing degradation, are accumulative in their or- 

 dinary action, when the material exposed to them is movable : 

 they are constantly piling up, while the upper waters are 

 eroding, and preparing material to be Carried off. The height 

 at which these two operations balance one another will be 

 the height, therefore, of the line of no degradation. As the 

 sea at low tide is mostly quiet, and the lower of the surging 

 waters swell on to receive the upper and parry the blow, and 

 moreover, there is next a return current outward, we should 

 infer that the line would be situated more or less above low 

 tide, according to the height' of the tide and the surges ac- 

 companying it. We are not left to conjecture on this point ; 

 for the examples presented by the shores of Australia and 

 New Zealand afford definite facts. Degradation has there 

 taken place sufficient to carry off cliffs of rock, of great ex- 

 tent ; yet below a certain level, the sea has had little or no ef- 

 fect. This height, on the eastern shores of Australia, is three 



