FORMATION OF CORAL REEFS AND ISLANDS. 239 
In the case of the coral island, the material of the coral 
platform is piled up by the advancing surges, and cemented 
through .the infiltrating waters. These surges, on reaching 
the edge of the shelf, break upon it with more or less force 
during low tide and the commencing rise; but later the 
waters swell over it before breaking, and thus throw a pro- 
tection about the exposed rocks; and as the tide continues to 
rise, they sweep over the shelf, but only clear it of sand and 
fragments, by bearing them to the beach on which they ex- 
pend their force. Where the tides are five to six feet in 
height, the shore platform of atolls is narrow. 
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. 
Il, 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. 229), 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. The seasons are 
not as unlike in the tropical] part of the Pacific. But in all 
seas there is a marked difference and in some stormy months 
