76 ON CORAL REEFS AND ISLANDS. 



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 ordinary 

 action, when the material exposed to them is movable : they are 

 constantly piling up, while the upper waters are rending and pre- 

 paring 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 arid New Zealand afford definite facts. Degradation has 

 there taken place sufficient to carry off cliffs of rock, of great 

 extent ; yet below a certain level, the sea has had little or no 

 effect. This height, at New Holland, is three feet above ordin- 

 ary low tide, and at New Zealand, about five feet. With regard 

 to the height varying with the tides, we observe that in the Pau- 

 motus, where the water rises but two or three feet, the platform 

 is seldom over four to six inches above low tide, which is pro- 

 portionally less than at New Holland and New Zealand, where 

 the tide is six and eight feet. From these observations, it ap- 

 pears that the height of ?io wave-action, as regards the degrada- 

 tion of a coast under ordinary seas, is situated near one-fifth tide, 

 in the Paumotus, and above half tide at New Zealand, showing 

 a great difference between the effect of the comparatively quiet 

 surges of the middle Pacific, and the more violent of New Zea- 

 land. Within the Bay of Islands, where the sea has not its full 

 force, the platform, as around the " Old Hat," is but little above 

 low water level. The exact relation of the height of the plat- 

 form to the height and force of the tides remains to be deter- 

 mined more accurately by observation. While, therefore, the 

 height of the shore platform depends on the tides, and the usual 

 strength of the waves, the breadth of it will be determined by 

 the same causes in connection with the nature of the rock- 

 material.* 



* On basaltic shores it is not usual to find a shore platform, as the rock scarcely 

 undergoes any degradation, except from the most violent seas ; such coasts are con- 



