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SCIENCE. 



[N. S. Vol. XI. No. 264. 



and others of less dimensions, considerable 

 sea rises under the pi-evailing trades. The 

 sea and wind generally follow the trend of 

 the shores, both in the lagoon and along 

 the sea face, so that the bars of beach rock 

 act like buttresses and collect material at 

 their inner and outer extremities, forming 

 the sand bars and islets which eventually 

 become the land rim of the lagoon. When 

 the material is not from local causes very 

 abundant, or is washed out over the flats, 

 there are fewer islands, and often these 

 are but mere islets or bars for long reaches 

 of the shore, forming the characteristic 

 weather faces of many of the lagoons. 



Many of the lagoons are filled with shoals 

 or ledges awash or a few feet above the sea- 

 level. These shoals are parts of the old 

 ledge which have not as yet been eroded, 

 and the disintegration of which has gone 

 far to supply material for the land of the 

 outer rims of the atolls. In Fakarava there 

 were no less than 36 islands and islets and 

 ledges, parts of a former great flat, now 

 broken up, existing parallel to the outer 

 reef flat about four miles in the lagoon. 

 Similar reef fiats exist in Tahanea, where 

 they form a secondary lagoon with two to 

 three fathoms of water, extending nearly 

 the whole length of the western face of the 

 atoll. There are several large islands on 

 this flat, and at high water they would ap- 

 pear as the islands and islets of Fakarava 

 do, as disconnected and planted in the 

 lagoon itself. A secondary lagoon also 

 exists in Eavahere and Anaa ; in both 

 these atolls the reef flat extends across one 

 extremity of the lagoon and does not ran 

 parallel to the longer shore- line of the atoll. 



The lagoons of these atolls have a gen- 

 eral depth of 13 to 20 fathoms. In some 

 cases they are, as is stated, somewhat 

 deeper (but there are no measurements), 

 the greater depths being 30 fathoms or 

 more, being due to orogenic conditions. 

 Some of the atolls are quite shallow, as at 



Matahi va as well as Pinaki , where the lagoon 

 is not more than two to three fathoms, and 

 Takume, where it is from five to six fathoms 

 deep. Some of the smaller islets we visited, 

 among which are Tekei, Aki-Aki and Nuku- 

 tavake, have no lagoons. The former has 

 a shallow sink in which fresh water col- 

 lects, but the rim is only very slightly 

 higher than the interior. The last two is- 

 lets are apparently depressed in the center, 

 three to four feet below the outer bank of 

 sand which forms the rim (about 10 to 12 feet 

 high) of the basin of the island. I was at 

 first inclined to look upon these islands as 

 examples of islands which had been cut 

 down to the level of the sea and subse- 

 quently been built up by beach rock and 

 sand in the manner described above. The 

 existence of extensive sand dunes on two 

 sides of the island at Pinaki, and of a large 

 dune (estimated to be 35 feet high) on the 

 south shore of ISTukutavake, seems to indi- 

 cate the possibility of there having been a 

 shallowlagoon occupying the center of Aki- 

 Aki and of Nukutavake, and that these 

 lagoons were gradually filled by the sand 

 dunes, much as Pinaki is filling now. 



At Pinaki (Whitsunday Island), there is 

 no doubt but that the lagoon is rapidly fill- 

 ing from the sand^ blown in by the dunes. 

 They are from 12 to 15 feet high and are 

 forcing their way in towards the lagoon, 

 killing the pandanus and whatever vegeta- 

 tion there is growing on the land rim of the 

 lagoon. The dunes have probably filled 

 also a second entrance to the lagoon, indi- 

 cated now only by a somewhat lower level 

 of the laud rim. Mr. Moore and Mr. 

 Townsend, who went ashore at Pinaki, re- 

 port that the lagoon is not more than three 

 fathoms deep ; they could wade over the 

 greater part of it. Mr. Alexander counted 

 no less than 116 islets in this small lagoon 

 — less than a mile in diameter — islets 

 formed of masses of dead Tridacna shells 

 thrown up on ledge rock, on the slopes of 



