On Coral Reefs and Islands. 35 
rent is obvious. The sea is constantly pouring water over the 
reefs into the harbor, 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 harbor-entrance, this being the 
only means of exit. This is a correct history of numerous 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 they change in direction with the 
tides.” The general mode of action, however, is the same. 
Acurrent of water of the kind here represented, will carry 
out much coral debris, and strew it along its course. ‘The trans- 
ported 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 harbors may gen- 
erally 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 vary- 
ing conditions, there may be growing corals over the bottom in 
some places and not in others; and the reefs may be distributed 
in patches, when, without such an influence, we should expect 
a general continuity of coral reef over the whole reef-grounds. 
The results from marine currents are often increased by waters 
from the island streams; for the coves, where harbors are most 
likely to be found, are also the embouchures of valleys and the 
streamlets they contain. The fresh waters poured in, add to the 
amount of water and increase the rapidity of the out-current. 
At Apia, Upolu, there is a stream thirty yards wide ; and many 
other similar instances might be mentioned. These waters from 
the land bring down also much detritus, especially during fresh- 
: é: p 
means by which freshwater streams contribute towards determin- 
ing the existence of harbors; for little is due to their freshening 
the salt waters of the sea. i £ 
The small influence of the last-mentioned ae one 
most commonly appealed to—will be obvious, when we consi 
the size of the streams of the Pacific islands, and the fact that 
fresh water is lighter than salt, nar ree instead of 8 nking, 
lows on over its surface. ‘The deepest rivers are seldom 0°C" 
six feet, even at their mouths; and three or four feet 1s 4 
Nie 
