AGASSIZ : FIJI ISLANDS AND COEAL REEFS. 11 



Reef can be traced as islands, islets, or negro-heads all along its line 

 for more than a thousand miles. Finally, in the description of the 

 islands of Fiji this substratum appears over and over again, either 

 composed of volcanic rocks, or of great tertiary limestone banks. No 

 better example can be found of the appearance of the substi-atum of 

 the recent reefs than in Kaneohe Bay, Oahu, at the Sandwich Islands, 

 where the reef is studded with islets and negro-heads consisting of 

 volcanic rocks. 



That corals grow in lagoons is well ascertained, and nowhere is it 

 better seen than in Fiji, where nearly all the islands enclosed by barrier 

 reefs are edged with fringing coral reefs. But why that should prevent 

 a lagoon from being formed I cannot see. A lagoon is not bounded 

 by a reef forming a closed wall rising well above the level of the sea. 

 The greater part of the i-eef of many a lagoon of an atoll or barrier reef 

 has from two to three fathoms of water upon it at high tide. The reef is 

 also riddled on all sides with narrow channels or openings with from one 

 to two fathoms or more at low tide, in addition to the wider and deeper 

 passages to leeward, through which access is gained into the lagoon. 

 But for all this the lagoon exists, while it may not have more than a 

 few fathoms in maximum depth. This, however, does not prevent the 

 coral heads on the inner slope of the reef from gradually becoming con- 

 nected witli the reef, and from encroaching little by little, but very 

 slowly, upon the outer margin of the lagoon to a depth of seven or eight 

 fathoms, at which the growth is checked either from the sediment accu- 

 mulating on the floor, or from the strength of the currents scouring the 

 bottom of the lagoon. The amount of dead coral which is ground up 

 upon a reef flat is considerable. Much of it is cemented together and 

 forms a breccia in the cavities of the coral heads, or in the open spaces 

 between them. Still more of it is changed into sand and mud, which 

 cover the floor of the lagoons of barrier reefs and of atolls, and finally 

 a quantity is carried off in solution after the dead coral has become 

 thoroughly rotten and crumbling. 



Darwin also visited the western side of Mauritius, where, he says : ^ "It 

 is probable that a reef on a shelving shore, like that of Mauritius, would 

 at first grow up not attached to the actual beach, but at some little 

 distance from it; and the corals on the outer margin would be the 

 most vigorous. A shallow channel would thus be formed within the 

 reef; and this channel could be filled up only very slowly with sediment, 



1 Darwin's Coral Reefs, 3d ed., 1889, p. 72. 



