20O CORALS AND CORAL ISLANDS. 



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 covered 

 with large fragments of the basaltic rocks. But on sandstone 

 shores, this gradual action keeps the platform of nearly uniform 

 breadth. Moreover, any uptorn masses thrown upon it, are soon 

 destroyed by the same action, and carried off; and thus the plat- 

 form is kept nearly clean of debris, even to the base of the cliff. 



It is apparent that one single principle meets all the various 

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

 sand-spits 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 to- 

 ward 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, sweep- 

 ing over the shelf, but only clear it of sand and fragments, 

 which they bear to the beach. 



The isolated blocks in the Paumotus which stand on the 

 platform, attached to it below, are generally most worn one or 

 two feet above high-tide level, a fact which corresponds with 

 the statement in a preceding paragraph with regard to the 

 height of the greatest wave-action. 



III. EFFECTS OF WINDS AND GALES. 



In addition to this ordinary wave-action, there are also more 

 violent effects from storms ; and these are observed alike on 

 the Australian shores referred to, and on those of coral islands. 

 Tlie waters as they move in, first draw away and then 

 drive on with increased velocity up the shallowing shores, or 

 under shelving layers, and thus they easily break off great 



