J. D. Dana, Esq., on Coral Reefs and Islands. 229 



of the wave is raised considerably above the general level of 

 the water. From those 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 advan- 

 cing 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. 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 

 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 

 distinguish 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 ordinary action, when the material exposed to them 

 is moveable ; they are constantly piling up, while the upper 

 waters are rending 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 accompanying it. We are not left to conjecture on 

 this point, for the examples presented by the shores of New 

 Holland and New Zealand, afford definite facts. Degrada- 

 tion has there taken place sufficient to caiTy off cliffs of rock 

 of great extent ;, yet, below a certain level, the sea has had 

 l^Jitle or no effect. This height, at New Holland, is three 



