112 CORAL FORMATIONS. 
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 explana- 
tion. The material of the coral platform is piled up by the advancing 
surges, and cemented through the infiltrating waters, which take lime 
into solution, and again distribute it. These surges, advancing to- 
wards 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 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. 
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 shores among cavities or under shelving layers, break 
and lift the rocks of the edge of the platform, and throw them on the 
reef. From the observations of Mr. Stevenson, cited in the note to 
page 106, 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 
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. 
