AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 137 



directions, and which has undoubtedly played a great part in the lifting 

 of the island masses and their subsequexat shaping to their present out- 

 lines. From this evidence I am inclined to think that the corals of to- 

 day have actually played no part in the shaping of the circular or irregular 

 atolls scattered among the Fiji Islands, that they have had nothing to do 

 in our time with the building up of the substructure of the barrier reefs 

 encircling either wholly or in part some of the islands, that their modi- 

 fying influence has been entirely limited in the present epoch to the 

 formation of fringing reefs, and that the recent corals living upon the 

 outer margin of the reefs, either of the atolls or of the barriers., form only 

 a crust of very moderate thickness upon the underlying base. This base 

 may be either the edge of a submarine flat, or of an eroded elevated 

 limestone, or of a similar substructure composed of volcanic rocks, the 

 nature of that base depending absolutely upon its character when ele- 

 vated in a former period to a greater height than it now has ; denudation 

 and erosion acting of course more rapidl}^ upon the elevated coralliferous 

 limestones than upon those of a volcanic character. It is therefore natu- 

 ral to find that the larger islands, like Kandavu, Ovalau, and Taviuni 

 (Plates 1, 3, 4, 7, 10, 11), are of volcanic origin, while the islands which once 

 occupied the area of the lagoons of JSTgele Levu, Il^anuku Reefs, Vanua 

 Mbalavu, the Argo Reefs, the Oneata, Yaugasa, Aiwa, Ongea, and Vatu 

 Leile clusters, were composed of elevated coralliferous limestones. They 

 have disappeared almost entirely, leaving only here and there a small 

 island to attest to the former existence of a more extensive elevated 

 limestone, once covering the whole area of what is now an atoll (Plates 

 1, 17, 18, 19-21). Smaller volcanic islands, like Matuku, Moala, Ngau, 

 Nairai, and Koro (Plates 1, 12, 13, 14, 16), also show the greater or 

 smaller extent to which each has been eroded after its elevation, being 

 least in Koro (Plate 3^) and Matuku (Plate 16), and somewhat more in 

 Moala (Plate 16) and Ngau (Plate 13), and still more in Nairai (Plate 

 14), while in such volcanic islands with atolls as Mbengha (Plate 8), 

 Wakaya, and Makongai (Plate 15) the denudation and submarine ero- 

 sion ^ has been still greater, the islands covering but a comparatively 



1 Dana (p. 230) accounts for the formation of the shore platform by the action of 

 the sea. We go a little further, and assign to the action of the breakers and of the 

 currents in carrying loose material to sea the formation of channels between the 

 outer reefs and the shore ; these become lagoons inside of barriers or encircling reefs, 

 and finally scoop out the lagoons of atolls. Dana (p. 181 ) insists fully as strongly as 

 Darwin upon the identity of origin of the encircling atoll reef and the outer reefs 

 enclosing high or low islands : " The lagoons are similar in character and position 



