CORAL-REEFS. 75 



ten or twelve feet, so that it offers a safe coasting channel 

 for boats. On the eastern and windward side of the island, 

 which is exposed to a heavy surf, the reef was described to 

 me as having a hard smooth surface, very slightly inclined 

 inwards, just covered at low-water, and traversed by gullies ; 

 it appears to be quite similar in structure to the reefs of the 

 barrier and atoll classes. 



The reef of Mauritius, in front of every river and 

 streamlet, is breached by a straight passage : at Grand Port, 

 however, there is a channel like that within a barrier-reef; 

 it extends parallel to the shore for four miles, and has an 

 average depth of 10 or 12 fathoms; its presence may 

 probably be accounted for by two rivers which enter at 

 each end of the channel, and bend towards each other. 

 The fact of reefs of the fringing class being always breached 

 in front of streams, even of those which are dry during the 

 greater part of the year, will be explained, when the con- 

 ditions unfavourable to the growth of coral are considered. 

 Low coral-islets, like those on barrier-reefs and atolls, are 

 seldom formed on reefs of this class, owing apparently in 

 some cases to their narrowness, and in others to the gentle 

 slope of the reef outside not yielding many fragments to 

 the breakers. On the windward side, however, of the 

 Mauritius, two or three small islets have been formed. 



It appears, as will be shown in the ensuing chapter, 

 that the action of the surf is favourable to the vigorous 

 growth of the stronger corals, and that sand or sediment, if 

 agitated by the waves, is injurious to them. Hence it is 

 probable that a reef on a shelving shore, like that of 

 Mauritius, would at first grow up, not attached to the actual 

 beach, but at some little distance from it ; and the corals 

 on the outer margin would be the most vigorous. A 

 shallow channel would thus be formed within the reef, and 



