252, CORALS AND CORAL ISLANDS. 
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; andthe coral débris 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 
divect 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. 
Although atolls in seas of moderate tides have the sta- 
bility stated on page 257, yet in those having tides of six 
feet or over and subject to the sweep of cyclones, they may 
find it difficult to stand their ground and repair losses above 
mean tide. But the larger reef islands may be increased in 
height by the more powerful agencies, as is well exemplified 
in the Bermuda Islands and the Bahamas. 


