70 FRINGING BEEFS. Ch. III. 



where, if as is probable the bottom of the sea has a 

 similar inclination, the coral would have no foundation 

 on which to become attached. A similar fact may 

 sometimes be observed even in reefs of the barrier 

 class, which follow much less closely the outline of the 

 adjoining land; as, for instance, on the S.E. and 

 precipitous side of Tahiti, where the encircling reef is 

 interrupted. On the western side of the Mauritius, 

 which was the only part I visited, the reef generally 

 lies at the distance of about half a mile from the 

 shore ; but in some parts it is distant from one to two, 

 and even three miles. Even in this last case, as the 

 coast-land is gently inclined from the foot of the 

 mountains to the sea-beach, and as the soundings 

 outside the reef indicate an equally gentle slope 

 beneath the water, there is no reason for supposing 

 that the basis of the reef, formed by the prolongation 

 of the strata of the island, lies at a greater depth than 

 that at which the polypifers could begin constructing 

 the reef. Some allowance, however, must be made for 

 the outward extension of a foundation formed of sand 

 and detritus, from the wear of the corals ; and this 

 would give to the reef a somewhat greater vertical 

 thickness than would otherwise be possible. 



The outer edge of the reef on the western or 

 leeward side of the island, is tolerably well defined, 

 and is a little higher than any other part. It chiefly 

 consists of large strongly branched corals of the genus 

 Madrepora, which also form a sloping bed some way 

 out to sea : the kinds of coral growing in this part 



