140 BULLETI>' : MUSEUM OF COMPAKATIVE ZOOLOGY, 



barrier reef stretching along the south coast of Viti Levu, especially at 

 Lauthala Bay, Suva (Plate 7), and the reef harbors and passages between 

 Suva and Serua (Plate 5), and out of the smaller atolls like Motua lai lai, 

 Thakau Leka leka, Thakau Momo, and others (Plates 12. 18, 21). 



The strength of the currents in the channel separating tlie barrier 

 reef and the shore has been noticed by Semper -^ and by Mobius ^ as 

 bearing an important part in Mauritius. There is in the Biologisches 

 Centralblatt, 1889-90, Bd. IX. p. 564, a short review of the third edi- 

 tion of Darwin's " Coral Pieefs," showing the principal points in the 

 discussion of the reefs to which Bouney has called attention. But this 

 discussion is mainly theoretical, and adds no new factors in the problem. 

 I would refer to what Gardiner ^ says regarding the conditions affecting 

 the growth of coral reefs in Fiji, where he shows the effect of tidal 

 currents in the passages of reefs and inside of reefs. Strong currents 

 prevent the coral larvae from fixing themselves in localities which they 

 scour, while the corals will thrive off the passages where the currents 

 have lost their strength. The navigators and naturalists of the U. S. 

 Exploring Expedition * frequently speak of the rapid outward current 

 passing through the openings of the reefs, especially during the ebbing 

 tide. Dana ^ had noticed the great strength of the tidal currents, and 

 he well explains by their action the great diversity of distribution of 

 material over the bottom of a lagoon or of a barrier reef channel. 



There seems to be no question that the action of the sea can cut out the 

 lagoons of barrier reefs and of atolls at the depths at which they have been 

 observed in the Fijis. Although there are individual atolls which show 

 depths of thirty-five to fifty fathoms and even more, these are excep- 

 tional depths, which are readily explained as due to other causes than 

 the scouring action of the sea. 



Admiral "Wharton ^ has given an excellent summary of cases showing to 

 what depths the action of the sea in motion may be felt to a sufficient ex- 

 tent to move material at depths of fifty to sixty fathoms. As he justly 

 says, " The effect [of the action of the waves in an otherwise deep sea 

 over which strong winds are continually blowing] will be to cut down an 

 island more or less rapidly, according to its constitution, to a very con- 

 siderable depth below the surface, the final result being a perfectly flat 

 bank." 



1 Natiirl. Existenzbeding. d. Thiere, Bd. II. (1880), chapters 7, 8. 



" Beitrage z. Meeres Fauna der Insel Mauritius, Berlin, 1880, p. 29. 



3 Loc. cit., p. 484. * Dana, p. 170. 



5 Loc. cit., p. 151. 6 Mature, Vol. LV. p. 392. 



