120 CORAL FORMATIONS. 
3. Harbours which receive fresh-water streams are more apt to be 
clear from sunken patches; and the same cause keeps open shallow 
passages to the shores, where there are shore reefs. 
It should be remembered, that while the effects from fresh-water 
streams are so trifling around islands, they may be of very wide in- 
fluence 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 respecting reefs around other 
lands, apply equally to atoll reefs. There are usually currents flow- 
ing to leeward through the lagoon, and out, over or through the 
leeward reef, and this action, as with the coral harbour, 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, and per- 
mits the waves in some parts to break over it; and the amount of coral 
debris they bear along will then be greatest. When a large part of 
the leeward reef is under water, or at low tide level, the waters may 
escape over the whole, and on this account we sometimes find large 
reefs without any proper channels. As the land to windward becomes 
raised throughout above the sea, and forms a continuous line which 
the waves cannot pass, the current is less perfectly 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 iden- 
tical with what has been explained. ‘The absence of coves of land to 
give force to the waters of the currents, and to direct their course, and 
the absence also of fresh-water streams, are the only modifying causes 
not present. It is readily understood, therefore, why lagoon entrances 
are more likely to become filled up by growing coral, than the pas- 
sages through barrier reefs. 
