There is nothing in the material within the lagoon to favor
solution more than in either of the other two regions; in
fact, the platform and bare reef are most exposed to the
action because of the small amount of living corals over
them. The outside, waters take up what they can through
the carbonic acid they contain, and supply thereby the wants
of the lime-secreting polyps, shells, etc., and carry on the pro-
cess of solidification in the debris ; the same waters move on
over the atoll reef and take up more lime as far as the acid
ingredient is present ; and then they pass to the lagoon for
work similar to that outside, with probably a diminished
amount of free carbonic acid, on account of the loss over the
reef -ground previously traversed.
The lagoon-basin is not, therefore, the part of the atoll
that loses most by solution, any more than by abrasion and
transportation. The outer reefs suffer the most ; and yet, if
the island is not subsiding at too rapid a rate, they keep ex-
tending and encroaching on the ocean, instead of wasting
through the drifting into the ocean at large of calcium car-
bonate in grains and solution, and the shore-platform also
preserves its unvaried level notwithstanding the daily sweep
of the tidal floods, and the holes that riddle its outer portion.
300 CORALS AND CORAL ISLANDS.
