38 PERCY SLADEN TRUST EXPEDITION. 
certain amount of quarrelling about domestic relations, which are decidedly mixed. ‘The 
islands are singularly healthy, though the manager keeps a good supply of medicines 
and acts as doctor in cases of necessity. 
Our time was very fully occupied while we were at Ile du Coin, as we examined and 
collected practically in every part of the 8 miles of continuous reef, on which it forms 
the central island. We devoted this special attention to it because, being largely open 
to the south and swept by strong currents, it might differ in some of its characters from 
the reefs of Salomon, Diego, and Egmont, and resemble the isolated reefs of the Great 
Chagos bank. In this we were not disappointed, since the reef appeared to be extending 
on either side at the expense of both ocean and lagoon. Its outer (seaward) edge 
consists of buttresses, 2-8 yards broad, covered with Lithothamnia, and divided one from 
the other by deep fissures 20 to 40 yards long, which gradually lessen in width and depth 
as they penetrate into the reef-flat. ‘The inner (lagoonward) edge is less irregular in 
outline and to a large extent is composed of massive coral-heads which have grown up 
and fused with the adjacent reef-edge, the interstices between them becoming filled up 
with sand and rubble. Off Mapous de I’He du Coin the inner edge is more or less 
consolidated by encrusting Lithothamnia, which are not present off He du Coin itself or 
further north. Indeed, the lagoon-slope there is quite irregular and supports corals of 
many kinds, such as Mussa, Madrepora, Stylophora, and Pocillopora, at depths of 5 to 10 
fathoms ; while in the shallower waters down to 4 fathoms dominate the massive corals 
Siderastrea, Orbicella, Goniastrea, Prionastrea, and above all Porites with its broad 
heads, 20 feet in diameter, decayed in the centre, and spreading outwards on all sides. 
The well-defined edge of the reef is formed by the growing together of these corals, the 
spaces left between them being filled up by various smaller species of reef-corals together 
with Willepora, Aleyonacea, and, further in, with Heliopora and Tubipora. 
On either side of the islands the reef-flats, each in accordance with its mode of 
formation, are outside a flat of solid rock and inside a conglomerate of huge blocks of 
decaying coral with patches of soft discoloured sand between. ‘The reef-edges on the 
two sides show their different composition, for while the outer, covered with hardy surf- 
loving Lithothamnia, steadily presses its way seawards against the mighty rollers which 
crash upon it, the inner with its corals does little more than hold its own against the 
far lighter waves which rise up in the shallow lagoon. A further difference exists in 
the complete continuity of the outer reef as against the numerous boat-channels and 
lesser passages on the lagoon-sides of most of the islands. It is through these passages 
that the tidal water rushes, bearing with it sand and mud from the reef in sufficient 
quantities to prevent any corals taking root. 
No definite boulder-zone exists on the lagoon side, though there are patches of corals 
thrown up off Mapous de I'Ile du Coin and again further to the east. It is, however, 
well marked along the whole length of the reef to seaward, and generally runs into the 
shores of the islands, or at least is connected with them by ridges and masses of rocks 
showing where the islands have once extended outwards. The south part of Monpatre has 
been washed back, but the north part still extends out to the boulder-zone. Within the 
last 20 years, however, the sea has cut a passage behind the latter diagonally through 
