On Coral Reefs and Islands. 39 
In recapitulation, the existence of passages through reefs, and 
the charge tet of coral harbors may be attributed to the following 
causes 
1. The configuration and character of the submarine land ;— 
corals not growing where the depth exceeds certain limits, or 
where there is no firm beesiiners for the plantation. 
The direction and force of marine currents with their trans- 
ported detritus ;—these currents deriving their course, as in other 
regions, from the features of the land, the form of the sea-bot- 
tom and the per and being sometimes increased in force by the 
contributions of island streams, which add to the detritus and to 
the weight of accumulating waters. 
3. Harbors which receive freshwater streams or submarine 
springs of freshwater, are more apt to be clear from sunken 
patches; and the same causes keep open shallow passages to the 
shores, where there are shore reefs 
It should be remembered, that while the effects from freshwater 
Streams are so trifling around islands, they may be of very wide 
influence on the shores of continents, where the streams are large 
and deep, and transport much detritus. ‘This point is illustrated 
ond. 
b. Atoll Recfs.—The remarks in the preceding pages respect- 
ing reefs around other lands, apply equally to atoll reefs. ‘There 
are usually currents flowing to leeward through the lagoon, and 
ont, over or through the leeward reef, and this action, as with 
the coral harbor, tends to keep open a leeward channel for the 
e of the water. This is the common explanation of the 
origin of the channels opening into lagoons. ‘These currents are 
Strongest when the windward reef is low, so as to permit the waves 
Mm some parts to break over it; and the coral debris they bear 
along will then be greatest. When a ange part of the leeward 
tinuous line which the waves cannot the current is less per- 
soctly sustained, being dependent entirely upon the influx and 
ux of the tide es; and the leeward channels, in such a case, 
icy gradually become closed. 
action of currents on atolls is, therefore, in every way 
identical with what has been explained. 'The absence of coves 
land to give force to the waters of currents, and to difect 
their course, and the absence also of freshwater streams, are 
only matin causes not present. It is readily . 
why lagoon entrances are more likely to became f 
up by gr growing coral, than the passages through barrier reefs. 
