Sect. I. THE GROWTH OF CORAL-REEFS. 
89 
it enters the sea only in small quantity and during a 
part of the year. No doubt brackish water would pre- 
vent or retard the growth of coral ; but I believe that 
the mud and sand, which is deposited, even by small 
rivulets when flooded, is a much more efficient check. 
The reef on each side of the channel leading into Port 
Louis at Mauritius, ends abruptly in a wall, at the 
foot of which I sounded, and found a bed of thick mud. 
This steepness of the sides appears to be a general 
character in such breaches : Cook , 1 speaking of one 
at Raiatea, says, ‘ like all the rest, it is very steep on 
both sides.’ Now, if it were the fresh water mingling 
with the salt, which prevented the growth of coral, 
the reef certainly would not terminate abruptly; hut as 
the polypifers nearest the impure stream would grow 
less vigorously than those farther off, so would the 
reef gradually thin away. On the other hand, the 
sediment brought down from the land would only 
prevent the growth of the coral in the line of its 
deposition, but would not check it on the side, so that 
the reefs might increase till they overhung the bed of 
the channel. The breaches are much fewer in number, 
and front only the larger valleys in reefs of the en- 
circling harrier class. They probably are kept open 
in the same manner as those into the lagoon of an 
atoll, namely, by the force of the currents and the 
drifting outwards of fine sediment. Their position in 
front of valleys, although often separated from the 
land by deep-water lagoon-channels, which it might 
1 Cook’s First Voyage, vol. ii. p. 271. (Hawkesworth’s edit.) 
