AGASSIZ: FIJI ISLANDS AND CORAL KEEFS. 103 



oil the western edge of a bank sloping to the eastward, the 100 fathom 

 line being more than half a mile from the eastern edge of the reef flat. 



A number of negro-heads, appai'ently coral, could be seen on the 

 western horn of the reef flat. Plate ill gives a good idea of the 

 appearance of the curve of the narrow reef flat of one of these smaller 

 atolls, with the sea breaking over the rim. 



LAGOONS OF ATOLLS. 



Dana denies that there is any connection between the chnnnels of 

 lagoons and prevailing currents, and asserts that the channels tend to 

 be closed by the increase of gi'owing corals. Certainly our experieuce 

 in Fiji could not indicate such a conclusion, nor are the lagoons in tlie 

 smaller islands without channels except where a closed ring of coral 

 sand islets has accumulated on the outer rim, — a rare occurrence, 

 and one where the lagoon was formed while open to the influx of the 

 sea. Some of the small islands, with lagoons which are dry, may be 

 elevated islands reduced to that stage by atmospheric agencies. 



While there may be a large amount of coral ooze and nmd deposited 

 in the lagoon, yet even in the lagoons mentioned by Dana he states 

 that the sea has access to them, and that they are remarkable for the 

 salinity of the water and the absence of growing corals within the 

 lagoon at high water.^ 



Dana says ^ that nine tenths of the atolls under six miles in length, 

 half of those between six and twenty miles, and the majority of all 

 atolls in the Pacific Ocean, have no entrances to the lagoon a fathom 

 deep, and the larger part of those included in each of these groups have 

 no open entrances at all. He further says that nine, ranging from 

 one and a half to three miles in the larger diameter of the reef, have 

 no lagoon, only a small depression in its place ; two of these take in 

 water at high tide and the rest are dry (namely, seven), certainly 

 a very small proportion, and that of diminutive atolls which give us 

 little information regarding the formation of the larger ones. Surely we 

 cannot reverse the process and let the formation of the large atolls 

 precede that of the smaller, as is suggested by Dana,^ for in that case 

 we should have around the small atolls the platforms or slopes which 

 have gradually been formed by the filling of the larger lagoon as sup- 



1 Dana, Corals and Coral Islands, p. 182. 



2 Loc. cit., p. 300. ^ Loc. cit., p. 302. 



