74 CORAL-REEFS. 



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. But 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 pro- 

 longation 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 the corals on a foundation of sand 

 and detritus, formed from their own wear, which 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 will be described in the ensuing 

 chapter. Between the outer margin and the beach, there 

 is a flat space with a sandy bottom and a few tufts of living 

 coral ; in some parts it is so shallow, that people, by avoid- 

 ing the deeper holes and gullies, can wade across it at low 

 water; in other parts it is deeper, seldom however exceeding 



