84 ON CORAL REEFS AND ISLANDS. 



In recapitulation, the existence of passages through reefs, and 

 the character 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 basement for the plantation. 



2. 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 reefs, 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 

 beyond. 



b. Atoll Reefs. — 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 

 out, over or through the leeward reef, and this action, as with 

 the coral harbor, tends to keep open a leeward channel for the 

 passage 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 

 in some parts to break over it ; and the coral debris they bear 

 along will then be greatest. When a large part of the leeward 

 reef is under water, or barely at the water's edge, the waters may 

 escape over the whole, and on this account we sometimes find 

 large reefs without any proper channels. As the land to wind- 

 ward becomes raised throughout above the sea, and forms a con- 

 tinuous line which the waves cannot pass, the current is less per- 

 fectly sustained, being dependent entirely upon the influx and 

 efflux of the tides; and the leeward channels, in such a case, 

 may gradually become closed. 



The action of currents on atolls is, therefore, in every way 

 identical with what has been explained. The absence of coves 

 of land to give force to the waters of currents, and to direct 

 their course, and the absence also of freshwater streams, are the 

 only modifying causes not present. It is readily understood, 

 therefore, why lagoon entrances are more likely to became filled 

 up by growing coral, than the passages through barrier reefs. 



