go CORAL-REEFS. 



corals, but not on the broken and projecting stumps, and 

 therefore, in the former case, the fresh growth of the coral 

 might be thus prevented. 



In the last chapter I remarked that fringing-reefs are 

 almost universally breached, where streams enter the sea. 1 

 Most authors have attributed this fact to the injurious 

 effects of the fresh water, even where it enters the sea only 

 in small quantity, and during a part of the year. No 

 doubt brackish water would prevent or retard the growth of 

 coral; but I believe that the mud and' sand which is 

 deposited, even by 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, 2 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, but 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 



1 Lieut. Wellstead and others have remarked that this is the case in 

 the Red Sea; Dr. Riippell {Reise. in Abyss., Band. i. p. 142) says that 

 there are pear-shaped harbours in the upraised coral-coast, into which 

 periodical streams enter. From this circumstance, I presume, we 

 must infer that before the upheaval of the strata now forming the 

 coast-land, fresh water and sediment entered the sea at these points ; 

 and the coral being thus prevented growing, the pear-shaped harbours 

 were produced. 



2 Cook's First Voyage, vol. ii. p. 271. — (Hawkesworth's Edit.) 



