551.96 174 [March 



II 



The Broadening of Atoll-islets 



A POETIC siniilo of Dana's compared an atoll to a garland 

 thrown upon the waters. But few are the atolls that 

 answer to this ideal ; in most the wreath encircling the lagoon is 

 twisted and torn to fragments, forming a more or less complete 

 chain of atoll-islets. 



That the shore line of these islets is not stationary has long 

 been recognised. By an extension of their length they may unite 

 into a complete ring, and further growth upon the inner sides may 

 result in the obliteration of the lagoon and the conversion of 

 the atoll into an island. 



Murray 1 allows that in small atolls the islets may thus by 

 ingrowtli till up the lagoon, yet he considers that in atolls over two 

 miles in diameter the lagoon would enlarge by solution, which 

 implies that the lagoon shores of the islets would be eaten back. 



This idea has not received support from those who have studied 

 coral reefs on the spot in either the Indian, Atlantic, or Pacific 

 Oceans. Guppy '^ has described in admirable detail the way in 

 which the large lagoon of Keeling Island is gradually disappearing, 

 Heilprin ^ has pertinently remarked of the Bermudas that whereas 

 we should expect an exposure of bare rock to occur in a basin 

 v)f solution, his dredgings invariably proved the entire tioor to 

 be covered with a thick deposit of ooze. To me * the wide lagoon 

 of Funafuti seemed on every side to be encroached upon by the 

 land. On the other hand, Bourne observed in Diego Garcia that 

 the lagoon made inroads on the land. 



While attention has thus been paid to the increase or decrease 

 of the land upon the lagoon shore of an atoll, less notice seems to 

 have been bestowed on the possibility of fluctuations on the outer 

 or peripheral coast. Some features of the Funafuti beach suggested 

 to me a seaward growth. 



Crossing the largest or windward islet from east to west, that is 

 from the ocean to the lagoon, there appears first a steep and 



^ Nature, vol. xlviii., 1893, p. 57(5. 

 -Guppy, Scottish Geogr. Mag., vol. v., 1889, p. iuZ. 

 •' Heilprin, "The Bermudas Islands," 1889, p. 44. 

 ■• Hedley, " The Atoll of Funafuti," 1896, p. 17. 



