FORMATION OF REEFS. 77 



It is apparent that one single principle meets all the various 

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

 sand-spit on others, and the coral-reef platform of others, require 

 but one explanation. The material of the coral platform is piled 

 up by the advancing surges, and cemented through the infiltra- 

 ting 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 fragments, which they bear to the beach. 



The isolated blocks in the Paumotus which stand on the plat- 

 form, 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. 



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, moving through greater depths, and driving on with in- 

 creased velocity up the shallowing shores among cavities or under 

 shelving layers, break and lift the rocks of the edge of the plat- 

 form, and throw them on the reef. From the observations of 

 Mr. Stevenson, cited in a note to a preceding page, it appears that 

 the force of the waves during the summer and winter months 

 differs at Skerryvore more than 1200 pounds to the square foot, 

 — in the former it averaging but 636 pounds, and in the latter 

 2086 pounds, while in storms it was at times equivalent to 6083 

 pounds. The seasons are not as unlike in the tropical part of 

 the Pacific. Still there must be a marked difference between 

 the ordinary seas and those during stormy weather. We have 

 therefore no difficulty in comprehending how the ordinary wave- 

 action should build up and keep entire the shore platform, while 

 the more agitated seas may tear up parts of the structure formed, 

 and bear them on to the higher parts of the island. Still more 

 violent in action are the great earthquake-waves, which move 

 through the very depths of the ocean. 



These principles offer an explanation also of the general fact 

 that the windward reef is the highest. The ordinary seas, both 

 on the leeward and windward sides, are sufficient for producing 

 coral debris and building up the reef, and in this work the two 

 sides may go on with almost equal rate of progress : consequently 

 we may often find no very great difference in the width of the 



sequently often covered with large fragments of the basaltic rocks. But on sand 

 stone 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 platform is kept nearly clean of debris, even to the 

 base of the cliff. 



