ORIGIN OF HARBOURS. 115 
nearly 1000 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 be- 
tween, an entrance for ships. The harbour ave- He 
rages ten feet in depth, and at the entrance is 
fifteen feet. In this harbour there is a remarka- 
ble out-current along the bottom, which during 
gales, is so strong at certain states of the tide, 
that a ship at anchor, although a wind is blowing 
directly in the harbour, often rides with a slack cable; and in more 
moderate weather the vessel tails out against the wind. ‘Thus when 
no current but one inward is perceived at the surface, there is an 
under current acting against the keel and bottom of the vessel, which 
is of sufficient strength to counteract the influence of the winds on the 
rigging and hull. The cause of such a current is obvious. ‘The sea 
is constantly pouring water over the reefs into the harbour, and the 
tides are periodically adding to the accumulation ; the indented shores 
form a narrowing space where these waters tend to pile up: escape 
consequently takes place along the bottom by the harbour-entrance, 
this being the only means of exit. This is a correct history of nume- 
rous cases about all the islands. In a group like the Feejees, where 
many of the islands are large, and the reefs very extensive, the 
currents are still more remarkable, and change in direction with the 
tides. ‘The general mode of action, however, is the same. 
A current of water of the kind here represented, will carry out 
much coral debris, and strew it also along its course. ‘The transported 
material will vary in amount from time to time, according to the force 
and direction of the current. It is therefore evident that the ground 
over which it runs is wholly unfit for the growth of coral, since 
zoophytes are readily destroyed by depositions of earth or sand, and 
require a firm basement to commence growth. The existence of 
an opening through a reef requires, therefore, no other explanation ; 
and it is obvious that harbours may generally be expected to exist 
wherever the character of the coast is such as to produce currents 
and give a fixed direction to them. ’ 
The currents about the reef-grounds west of the large Feejee 
Islands, aid in distributing the debris both of the land and the reefs. 
In some parts the currents eddy and deposit their detritus; in others 
they sweep the bottom clean. Thus, under these varying conditions, 
there may be growing corals over the bottom in some places and not 
HARBOUR OF APIA, UPOLU. 
