70 
FRINGING -REEFS. 
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 ; hut 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 outw 7 ard 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 growung in this part 
