I 



J. D. Dana, Esq., on Coral Beefs and Islands. 231 



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. 



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 increased velocity up the shallowing 

 shore, among cavities or under shelving layers, break and 

 lift the rocks of tlie edge of the platform, 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 lb. to the square foot — in the 

 former averaging but 636 lb., and in the latter 2086 lb. ; 

 while in storms it was at times equivalent to 6083 lb. 

 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 struc- 

 ture 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 in 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 leeward and windward reefs, 

 especially as the wind for some parts of the year has a course 

 opposite to its usual direction. But seldom, except on the 

 side to windward, is a sufficient force brought to bear upon 

 the edge of the platform, to detach and uplift the larger coral 

 blocks. The distance to which the waves may roll on with- 

 out becoming too much weakened for the transportation of 

 uptorn blocks, will determine the outline of the forming land. 



