But on sandstone shores, if the rock is not too firm to yield
sensibly under the stroke of the breakers, this gradual action
keeps the platform of nearly uniform breadth. Moreover,
any masses torn from the edge of the platform and thrown
upon it by storm waves, or the heavier earthquake waves,
may be soon destroyed by the same action, and carried off ;
and thus the platform may be kept nearly clean of debris,
even to the base of the cliff.
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.
