AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 103 
on 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, apparently coral, could be seen on the 
western horn of the reef flat. Plate 111 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 channels of 
lagoons and prevailing currents, and asserts that the channels tend to 
be closed by the increase of growing corals. Certainly our experience 
in Fiji could not indicate such a conclusion, nor are the lagoons in the 
smaller islands without channels except where a closed ring of corai 
sand islets has accumulated on the outer rim,—a rare occurrence, 
and one where the Jagoon 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 mud 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 Loe. cit., p. 300. 3 Loe. cit., p. 302. 
