234 CORALS AND CORAL L8LANDS. 



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 Paumotus, 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 Austra- 

 lia 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 Paumotus, and above 

 half-tide at New Zealand, showing a great difference between 

 the effect of the comparatively quiet surges of the middle Pa- 

 cific, 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, and the force of wave- act ion, remains 

 to be determined more accurately by observation. While, 

 therefore, the height of the shore platform depends on the 

 tides, and the degree of exposure to 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 consequently often cov- 

 ered with large fragments of the basaltic rocks. But on sand- 

 stone shores, this gradual action keeps the platform of nearly 

 uniform breadth. Moreover, any up torn 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. 



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

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



