236 Prof. A. Agassiz on the 



points or spurs of the larger islands also bears witness to tlie 

 great length of time during which action of the sea necessary 

 to bring about their separation has been at work : adding to 

 this the fact that we are in a region of a former powerful and 

 extensive volcanic activity, the traces of which can still be 

 seen in all directions, forces which have undoubtedly played 

 a great part in the lifting of the island masses and their subse- 

 quent shaping to their present outlines. 



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 ; 

 furthermore, that they have had nothing to do in our time with 

 the building-up of the barrier reefs surrounding either wholly 

 or in part some of the islands ; I also believe 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 reefs either of the atolls or of the barriers 

 form only a crust of very moderate thickness upon the under- 

 Iving base. This base may be either a flat of an eroded 

 elevated reef or of a similar substructure of volcanic rocks, 

 the nature of that base depending absolutely upon its character 

 when elevated in a former period to a greater height than it 

 now occupies. 



Denudation and erosion act of course more rapidly upon tlie 

 elevated reef-rocks than upon those of a volcanic character. 

 It is therefore natural to find that the larger islands like 

 Kandavu, Taviuni, and Ovalau are of volcanic origin, while 

 the islands which once occupied the area of the lagoons of 

 l^gele Levu, of the Nanuku reef, of Vanua Mbalavu, of the 

 Argo reefs, of the Oneata, Yangasa, Aiwa, Ongea, and 

 Vatu Leile clusters, being elevated coral-reefs, have disap- 

 peared almost entirely, leaving only here and there a small 

 island to attest to the former existence of the more extensive 

 elevated reef once covering the whole area of what is now an 

 atoll. Smaller volcanic islands like Matuku, Moala, Ngau, 

 Nairai, and Koro also show the extent to which each island 

 has been eroded after its elevation, the erosion being least 

 in Koro and Matuku, somewhat greater in Moala and Ngau, 

 and still greater in Nairai. In such atolls enclosing volcanic 

 islands like Mbenga, Wakaya, Makongai, the erosion and 

 denudation have been still greater, these islands covering but 

 a comparatively small part of the area once occupied by the 

 island originally covering the area of the lagoon. Denudation 

 and erosion have been still more active in the Ringgold 

 Islands, in the Kimbobo cluster, and in Komo, and it may 

 have gone so far as to leave no trace in an atoll to indicate 



