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AGASSIZ: FIJI ISLANDS AND CORAL REEFS. LES 
The barrier reef flat north of Tomberua Passage and south of Rat Pas- 
sage is in many places nearly three miles wide, with an average width of 
about two miles. The wide reefs of Serua and of Rovondrau Bays 
(Plate 5) are covered with flourishing patches of corals; what their 
substratum is I am unable to determine from the absence of negro-heads 
on the outer reef flats. Judging however from the nature of the head- 
lands and of the rocks cropping out all along the shore between Serua 
and Suva, it is probably of the “ soapstone,” —the bedded volcanic mud 
so characteristic of the shores of Viti Levu both east and west of Suva, 
with the exception of a small key probably of elevated limestone on the 
end of Rovondrau Reefs (Plate 5). 
To the east of Namuka Harbor, a small reef harbor formed like the 
Navua and the Tongoro pass through the outer reef flats, and the elevated 
limestone again forms the substratum of the reef flats. The reef is here 
a fringing reef, which can be navigated by boats near the shore after half 
tide, and the outer face is covered with large negro-heads consisting of 
elevated coralliferous limestone. West of the entrance to Suva Harbor 
(Plate 5) the reef occurs in successive stretches of fringing or of barrier 
reef outside of fringing reefs, forming in some cases small reef harbors, 
like Namuka, Nanggara, Vatuloa, Navua, Serua, or, opposite the mouth 
of the Navua River, a large open bight, Rovondrau Bay. 
The passages through the Viti Levu barrier reef and its principal 
reef harbors, Suva among others, are generally opposite the mouth of 
some river or creek, through which considerable fresh water carrying 
silt flows ; this is especially the case of the Rewa River, which brings 
through its many mouths an immense amount of silt and detritus, and 
has formed a great delta. Both Darwin and Dana have attributed the 
formation of reef harbors largely to local currents, to the presence of 
fresh water, and to the action of silt. The bringing down of detritus, 
especially in the rainy season, and its deposition in channels will keep 
the bottom clear of corals, and thus form passages or reef harbors. 
The extensive reef which protects the harbor of Rewa has been formed 
in part by thé erosion of the softer volcanic beds, and in part by the 
deposition of the silt brought down by the Rewa and the cutting out 
of a barrier channel behind the reef. 
To the west of Serua the barrier reef disappears (Plates 5, 6), and the 
shore line is now composed of hard volcanic rocks which are not easily 
decomposed, or consists of the soft and easily disintegrated volcanic mud 
strata extending as a general thing from Mbau to Serua. Although 
here and there the harder rocks push out to the shore, where they are 
