AGASSIZ: FIJI ISLANDS AND COEAL EEEFS. 115 



The barrier reef flat north of Tomberua Passage and south of liat 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 luiable 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 ci'opping out all along the shore between Serua 

 and Suvo, it is probably of the " soapstone," — the bedded volcauic 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 likb 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 bv 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 Yiti Levu barrier reef and its principal 

 reef harbors, Suva among others, are generally opposite the mouth of 

 some river or creek, tlirough wliich considerable fresh water carrying 

 silt flows ; this is especially the case of the Rewa River, whicli 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 the 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 



