114 CORAL FORMATIONS. 
The lagoons in coral islands are constantly receiving more or less 
debris from the reefs; and patches of growing coral within also tend 
to fillthem up. But the effect is slow in its progress, and none but 
islands of small size show any approximation, as before stated, to an 
obliteration of the lagoon. 
2. CAUSES MODIFYING THE FORMS AND GROWTH OF REEFS. 
Coral reefs although (1) dependent on the configuration of the sub- 
marine lands for many of their features, undergo various modifications 
of form or condition through the influence of extraneous causes, 
such as (2) unequal exposure to the waves ; (3) oceanic or local currents ; 
(4) presence of fresh or tmpure waters. In briefly treating of these 
topics, we may consider /irst, reefs around high islands, and nezt atoll 
reefs. The effect of the waves on the different sides of reefs has 
already been considered, and we pass on, therefore, at once to the 
influence of oceanic or local currents, and fresh or impure waters. 
a. Barrier and Fringing Reefs. 
The existence of harbours 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 towards the same end, but much less than has been usually im- 
puted to their action.* 
There are usually strong tidal currents through the reef channels 
and openings. ‘T’hese currents are modified in character by the out- 
line of the coast, and are strongest wherever there are coves or bays 
to receive the advancing tides. The harbour of Apia, on the north 
side of Upolu, affords a striking illustration of this general principle. 
The coast at this place has an indentation 2000 yards wide and 
* The view here supported, is nearly identical with that presented by Mr. Darwin, 
(op. cit. page 68.) The arguments given were, however, written out (in 1840), before 
his descriptions of coral islands were known to me. This fact may give additional 
weight to the opinions, inasmuch as they are therefore the conclusions of independent 
observers, and are substantiated by a distinct set of observations. 
