AGASSIZ: FIJI ISLANDS AND CORAL REEFS. ci! 
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 substratum 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 
voleanic 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 reef 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 with 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. 
