418 Notices of Memoirs — Professor Alexander Agassiz — 



Ngau, and still more in Nairai ; while in such volcanic islands with 

 atolls as Mbengha, Wakaya, and Makongai the denudation and 

 submarine erosion have been still greater, the islands covering but 

 a comparatively small area of that once covered by the island 

 originally occupying the area of the lagoon ; this denudation having 

 been carried to a still greater extent in the Kimbombo cluster, in 

 Komo, and the islands of Duff Reef. This process of denudation and 

 submarine erosion may have gone so far as to leave no trace in an 

 atoll of its volcanic or of its limestone (elevated) origin, its shape 

 to-day being entirely due to mechanical action, and having nothing 

 to do with the growth of the corals which have found a footing upon 

 the flats due to submarine erosion and to denudation and to the 

 action of the atmosphere and of the sea. 



" It seems to me as if the position of an island left on the western 

 or lee edge of a lagoon depended upon the original position of its 

 highest point. This appears in the case of Makongai and Wakaya. 

 The crest of the former was probably near the eastern edge, while 

 the highest point of Wakaya was perhaps nearest the western side of 

 the original island. Similarly the highest summit and ridge of 

 Vatu Leile, if our views are correct, was on the western face of the 

 original land mass. The highest ridge of Eambe lies on the north- 

 western side of the submarine plateau ; the islands of Budd Eeef 

 indicate its highest land to have been on the northern part of the 

 plateau. 



" Admiral Wharton has suggested ' the cutting down of volcanic 

 islands by the action of the sea, and that this action has a far greater 

 share in furnishing coral foundations than has been generally 

 admitted.' From our experience in Fiji we may safely modify this 

 to the cutting down, not only of volcanic islands, but also of other 

 elevated islands, and their cutting down not only by submarine 

 erosion but also by denudation and atmospheric agencies, and thus 

 preparing the foundations upon which recent corals have established 

 themselves. Add to this the elevation of banks composed of volcanic 

 rocks or of sedimentary rocks up to heights at which corals or 

 •corallines can begin to gi'ow, and we have in addition to their 

 increment in height from the increase due to pelagic organisms and 

 the decay of other calcareous invertebrates living upon their surface 

 all the elements needed for the preparation of a set of foundations 

 from very different causes. 



"I have already on other occasions called attention to the powerful 

 scouring effect produced upon the interior of an atoll or lagoon, or 

 the channel of a barrier reef, by the mass of water poured into it 

 from all sides as the huge ocean swells break over the outer rim. 

 This mass of water can find no outlet against the incessant swell ; 

 it must escape to leeward through the openings in the outer reef 

 flats, or laterally over the low parts of their outer edges. It will be 

 noticed that the openings are usually on the west face of the atoll, 

 the direction in which the prevailing trades drive the water of the 

 lagoon. The water becomes charged with particles of lime or of 

 other material, and we soon have all the elements of a modified 



