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



feet above ordinary low tide, and at New Zealand, about five 

 feet. With regard to the height varying with the tides, we 

 observe that in the Panmotus, where the water rises but two 

 or three feet, the platform is seldom over four to six inches 

 above low tide, which is proportionally less than at New 

 Holland and New Zealand, where the tide is six and eight 

 feet. From these observations, it appears that the height of 

 no wave action, as regards the degradation of a coast under 

 ordinary seas, is situated near one-fifth tide in the Pan- 

 motus, and above half tide at New Zealand, shewing a great 

 difference between the eff*ect of the comparatively quiet surges 

 of the middle Pacific, and the more violent of New Zealand. 

 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 platform to the height and force of the tides, remains to 

 be determined more accurately by observation. While, there- 

 fore, 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.* 



It is apparent that one single principle meets all the va- 

 rious cases. The rocky platform of some sea shores, the low 

 tide sand-pit on others, and the coral-reef platform of others, 

 require but one explanation. The material of the coral plat- 

 form is piled up by the advancing surges, and cemented 

 through the infiltrating waters. These surges, advancing 

 towards the edge of the shelf, swell over it before breaking, 

 and thus throw a protection about the exposed rocks ; and 

 as the tide rises, this protection is complete. They move on, 

 sweeping over the shelf, but only clear it of sand and frag- 

 ments, which they bear to the beach. 



The isolated blocks in the Panmotus which stand on the 



* 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 consequently often covered with large fragments of the basaltic rocks. 

 But on sandstone shores, this gradual action keeps tlie jjlatform of nearly uni- 

 form breadth. JSIoreover, any uptorn masses thrown upon it, are soon destroyed 

 by the same action and carried off; and thus the platform is kept nearly clean 

 of debris even to the base of the cliff. 



