FORMATION OF CORAL REEFS AND ISLANDS. 245 
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 wit- 
ness, 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 char- 
acter 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 

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 har- 
bor 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 
