AGASSIZ: FIJI ISLANDS AND COKAL EEEFS. 139 



southern edges of the laud mass (Plate 22). In Yangasa the south- 

 eastern and western faces were probably the highest land of the group 

 (Plate 22). 



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 founda- 

 tions 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 grow, 

 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 ui 

 a set of foundations from very ditierent 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 pai-ts 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 prevail- 

 ino- ti-ades drive the water of the lagoon. The water becomes charjred 

 with particles of lime or of other material, and we soon have all the 

 elements of a modified gigantic pothole, from which the churned mate- 

 rial - is carried out by the currents flowing through the entrances iiito 

 the lagoon. It has long been known that there is a violent rush of 

 water out of the lagoons, the velocity attained reaching sometimes four 

 to five knots. In Fiji I have noticed these powerful currents flowing 

 out of the passages leading into the lagoons of Fulanga (Plate 22), of 

 Xgele Levu (Plate 17), of AVailangilala (Plate 18), of Vatu Leile (Plate 9), 

 uf Totoya (Plate 23j, and racing along the interior channels of the great 



1 " The Foundation of Atolls," Nature, February 25, 1897, p. 391. 



2 Material derived mainly from the mechanical disintegration of the corals or 

 substratum forming the surface of the reef, and also in part from the chemical dis- 

 integration due to the sea water at work to rot and dissolve the limestones of the 

 reef. 



