FORMATION OF CORAL REEFS AND ISLANDS. 235 



tide sand-spit 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. The 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 

 rocks from the edge of the platform, and throw them on the 

 reef. From the observations of Mr. Stevenson, cited on a pre- 

 ceding page (p. 225), it appears that the force of the waves 

 during the summer and winter months differs at Skerryvore 

 more than 1,200 pounds to the square foot, — in the former it 

 averaging but 636 pounds, and in the latter 2,086 pounds, 

 while in storms it was at times equivalent to 6,083 pounds. 

 The seasons are not as unlike in the tropical part of the Pa- 

 cific. Still there must be a marked difference between the or- 



