AGA.SSIZ : FIJI ISLANDS AND COEAL 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 la<''oons of 

 elevated ^tolls. 



I would refer to the description of such islands in Fiji as Mango, 

 Kambara, Tuvutha, Naia\i, Fulanga, and others, as evidence supporting 

 the explanation here given of the formation of the so called elevated 

 iitoUs. or course I do not mean to assert tliat an atoll cannot be 

 elevateil as such, nor tluit such atolls may not exist. I merely wish 

 to assert that the suuunit 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 tlie Yaugasa. 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 growtli of corals, and has generally been taken from old and unre- 

 liable soundings, as has been stated by Admiral Wliarton^ and othei's. 

 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 tlie underlying sub- 

 stratum, which may or may not be steep, and tlie 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 tlieir great thickness has been formed by sub- 

 sidence, and that tiiis 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- 

 pheric erosion. 



2 Nature, June 19, 1800, p. 172. 



