248 CORALS AND CORAL ISLANDS. 



II. ATOLL REEFS. 



The remarks on the preceding pages, respecting 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, the waves with the rising tide 

 dashing over the windward side, and keeping up a large sup- 

 ply, which is greatly increased in times of storms ; and this ac- 

 tion 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 strong- 

 est when a large part of the windward reef is low, so as to 

 permit the waves 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 large 

 reefs sometimes have no proper channels. When the land 

 to windward becomes raised throughout above the sea, so as 

 to form a continuous barrier which the waves cannot pass, 

 the current is less perfectly sustained, since it is then 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 fresh- water streams, 

 are the only modifying causes not present. It is readily un- 

 derstood, therefore, why lagoon entrances are more likely to 

 become filled .up by growing coral, than the passages through 

 barrier reefs. 



