410 PERCY SLADEN TRUST EXPEDITION 



a few rocks are still left standing well away from the land ; it is very shallow (2 fathoms), 

 but yet, owing to the rapidity of the current, carries a large bulk of water. An important 

 point to notice is that the fringing reef is not broken by a channel opposite the pass and 

 the escaping water therefore has to spread over the reef instead of flowing directly into 

 the ocean. There is evidence however that the reef is being worn away opposite the pass 

 and in time doubtless will be divided by a channel. 



Stage 3. The third stage is shown by Passe Johnny dividing Polymnie and Malabar 

 Islands, and Passe Dubois, the oldest of the " Western Channels." They only differ from 

 Passe Femmes in being deeper (4 — 6 fathoms) and in continuing through the fringing reef 

 to the sea. 



Stage 4. The final stage is represented by Grande Passe and Passe Houareau, which 

 differ from passes in stage 3 in the channel being deeper (Grande Passe 10 — 14 fathoms, 

 Passe Houareau 8 fathoms) and further in that they are fringed at the sides by a reef, 

 which exactly resembles the fringing reef of the whole atoll. This last point is of 

 considerable interest, for from large rocks of elevated coral limestone, still in situ, it was 

 abundantly clear that the reef on either side of the pass had not been formed by the 

 active growth of corals, but was in reality a ledge of elevated coral rock and the base 

 of land now washed away. 



As soon as the rock of the land-rim by a pass has been cut down below low water- 

 mark it is colonised by corals and lithothamnia, which protect it from further erosion, 

 while above low water-mark it is unprotected, and therefore the reef by the pass is due to 

 the difference in rate of erosion above and below low water-mark. From this we see that 

 the channel of a pass may speedily reach a limit in width, yet the reef will increase in size 

 continually at the expense of the land. Thus ultimately the islands will become separated 

 by increasing stretches of reef, a point which is of great assistance in considering the 

 present state of the atoll of Cosmoledo, and also the formation of the fringing reefs of all 

 the islands of the series. 



Before proceeding it may be well to say a few words as to the distribution of life 

 in the lagoon and passes, though for obvious reasons it was impossible to make complete 

 collections of the marine as well as of the land-fauna. Collections were made of the more 

 sedentary forms, and for details the reader must be referred to the papers by the 

 specialists who have kindly consented to work them out. In general, it may be stated 

 that the lagoon was extremely barren, a fact which is undoubtedly explained by the 

 universal presence of mud in the water. The swamps themselves were excessively foul, 

 and consequently no surprise was felt that the fauna was so limited. 



Live corals in the lagoon were almost limited to the regions of the passes, though 

 a few colonies of Porites were found along the various channels. In the neighbourhood 

 of Grande Passe is a definite flourishing reef, which is almost dry at low spring tides. 

 It is divided by numerous channels leading up to the pass, but does not seem to have 

 the power to encroach on them. The reef itself contained a uniformly varied assortment 

 of corals, although Madrepora of a branching fades in places formed definite fields to the 

 exclusion of other forms. As compared with other reefs visited Tubipora was unusually 

 common. Corals from this reef included species of Millepora, Tubipora, Heliopora, 



