AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 75 
formed by the carrying off the limestone in solution in the pools or ponds 
in the summit basins, — basins which with age become deeper and deeper, 
forming thus depressions which in some instances have been mistaken, 
in the one case for extinct craters, in the other for the lagoons of 
elevated atolls. 
I would refer to the description of such islands in Fiji as Mango, 
Kambara, Tuvutha, Naiau, Fulanga, and others, as evidence supporting 
the explanation here given of the formation of the so called elevated 
atolls. Of course I do not mean to assert that an atoll cannot be 
elevated as such, nor that such atolls may not exist. I merely wish 
to assert that the summit basins of islands formed of elevated cor- 
alliferous Tertiary limestones in Fiji are islands in the first stage of 
disintegration, passing gradually from such types as Naiau? and the 
like to islands like Mango, then to Fulanga, next to the Yangasa cluster, 
then to Ngele Levu, and finally to Motua lai lai and the like. 
The steepness of the slopes of coral reefs has been assumed to be due 
to the growth of corals, and has generally been taken from old and unre- 
liable soundings, as has been stated by Admiral Wharton? and others. 
The great depths are generally soundings so far off from the coral islands 
as not to give any accurate information. 
The actual slopes of coral reefs which have been measured are very 
few, and the slopes given are invariably those of the underlying sub- 
stratum, which may or may not be steep, and the inclination of which 
has no bearing on the angle of the steep outer slope of recent coral 
reefs, which usually reaches only to the very moderate depth of twelve 
to fifteen fathoms, 
It seems to me that the calculations which have been made by Dar- 
win in regard to the thickness of coral reefs, depending as they do upon 
the assumption that their great thickness has been formed by sub- 
sidence, and that this thick mass rests upon the continuation of the 
inner land slope under the coral reef, are assumptions which cannot be 
proved, or have not been proved. We should get a totally different 
result, that of a comparatively thin crust, if we assume that the land 
slope commences at or near the outer edge of the reef wherever the out- 
ermost negro-heads have been found. And in a great many cases, the 
steepness of the sea face is no greater than the slope of the mountains 
of the adjoining land. I have already called attention to the fact that 
1 Naiau itself having become depressed in the centre by chemical and atmos- 
pherie erosion. 
2 Nature, June 19, 1890, p. 172. 
