THE ATOLL OF MINIKOI. 33 



is what is really happening, since Dr Guppy ' has brought forward a great mass of cir- 

 cumstantial evidence to show that it is in this way that secondary atoUons are formed 

 reclaiming ultimately large areas from the sea. The southern half of Minikoi too would 

 seem to be absolutely similar to that of the Cocos-Keeling atoll described by Guppy. The 

 rocky seaward rim of Minikoi extends at Rocbera point directly into the boulder zone 

 of the reef, many long masses of rock more or less undermined marking its transition. 

 The rocky area before this had from the lighthouse westward been gradually narrowing, 

 and at the point the beach has a small cliff above with undermined or fallen bushes, 

 which conclusively prove its washing away. Just behind Rocbera is a small marsh or 

 kuW, covered with reed, its floor below the high-tide limit. Tunda point does not extend 

 directly into the lagoon, but a little to the north-west. Round it the water is deeper 

 close to the shore than further out towards Wiringili, and the greater part of the bottom 

 here is covered with a branching species of Pontes, right to the base of the beach, some 

 being thrown up at the point. North of the kidi all is sand, the same indeed as forms 

 the lagoon beach at Moli-Mati point. If some of the beach sand between Rocbera and 

 Tunda points (not a mere surface scraping) be sifted to remove the smallest particles, it 

 will be seen to be essentially the same. The sand carried over the reef in this position 

 is largely foraminiferal, while that on the beach is formed of finely triturated coral par- 

 ticles. It follows then that Tunda point must originally have been a lagoon formation. 

 Behind this jwint on each side the shores show a marked loss. The land is gradualli/ 

 narrowing, the sea, will sooner or later break aa'oss, the corals round the point will be silted 

 up and killed ; finally the ivhole ivill be washed away. Moli-Mati will probably, unless man 

 interferes, form such another point in the not very distant future. 



Before discussing the formation of the conglomerate, it is important to consider the 

 action — as deduced from observation — of the various agencies, to which a reef-rock, such as I 

 consider this rock to be, may be subjected above and below the sea. A reef, freely exposed to 

 the sea, may be compared in structure to a sponge, a series of hollow cavities joined together 

 by canals. Corals grow leaving cavities and spaces, which become silted up to some extent 

 with sand, composed of their own fragments and Foraminifera. These spaces are next bridged 

 over partially by means of the corals themselves, but more particularly by Lithothamnion and 

 Polytrema,^ (Foraminifera). When the cavities are once completely closed in from the sea 

 by organic growths, there is practically no circulation of water in them, causing any change; 

 boring and other organisms are killed, or depart for the more surface growths. As long 

 as there is a circulation of water, a continual solution and redeposition of the lime'' goes on. 

 The sand, being loose, is ground down into fine particles, which naturally, presenting a 

 relatively greater surface, would be first dissolved. The deposition would tend to occur 

 within the pores of the corals, where the change of water is less, and there is a fixed 

 support; the lime deposited follows the crystalline form of the coral. So long as the coral 

 skeletons are protected by organic growths, none of these changes can go on, but boring 



1 " The Cocos-Keeling Islands," Scot. Geo. Mag. (1889). vol. 4.3, p. 442, 1888). 



- I propose to employ this term for all such marshes ' This form gradually increases in importance to a limited 



of atoll islands, borrowing the name from the Maldivan, depth. 



whence is also derived the name atolu or atoll. The term ■* For the sake of brevity I use this term throughout 



appears to me preferable to barachois, which is used for instead of calcium carbonate, or bicarbonate. 

 such by G. C. Bourne in speaking of Diego Garcia (P. R. S., 



G. ' 5 



