Corals and coral islands.

FORMATION OF CORAL REEFS AND ISLANDS.

243

I. BARRIER AND FRINGING REEFS.
The existence of harbors about coral-bound lands, and of
entrances through reefs, is largely attributable to the action of
tidal or local marine currents. The presence of fresh-water'
streams has some effect toward the same end, but much less
than has been supposed. These causes are recognized by
Mr. Darwin in nearly the same manner as here: yet the
views presented may be taken as those of an independent witness, as they were written out before the publication of his
work.
There are usually strong tidal currents through the reef
channels and openings. These currents are modified in character by the outline of the coast, and are strongest wherever
there are coves or bays to receive the advancing tides. The
harbor of Apia, on the north side of Upolu, affords a striking
illustration of this general principle. The coast at this place
/ 1Z
1. 71
HARBOR OF APIA, UPOLU.
has an indentation 2,000 yards wide and nearly 1,000 deep,
as in the accompanying sketch, reduced from the chart by the
Expedition. The reef extends from either side, or cape, a mile
out to sea, leaving between an entrance for ships. The harbor averages ten feet in depth, and at the entrance is fifteen
feet. In this harbor there is a remarkable out-current along
the bottom, which, during gales, is so strong at certain states