J. S. Gardiner — Origin of Coral Reefs. 209 



of the changes in progress and here I have a firmer basis of 

 fact. First, it is obvious that change depends largely on the 

 rate of growth of reef-corals. Estimates of all sorts had been 

 made previous to my expedition, but none were founded on 

 other than isolated growths. I was, however, fortunate enough 

 to obtain forty coral colonies weighing 21,961 grams and measur- 

 ing 13,026 cubic cms. from an area of four square yards, which 

 had been absolutely cleared less than three years before. The 

 area being considerably enclosed was by no means a rich one, 

 and the corals had each to become aflSxed and to start from a 

 larva, but against this the position was in shallow water where 

 the full effect of sunlight would have been felt. According 

 to one method of estimating I conclude that tlie growth of a 

 reef would be upwards of 15 fathoms in 1,000 years, whilst 

 according to another method, which assumes all conditions to 

 be good, such as on the seaward side of a reef or on an open 

 bank, it would be upwards of 25 fathoms. Whatever estimate 

 may be correct, the facts show that the rate of growth of a 

 reef is relatively rapid, and that, unless checked in some way, 

 any coral-covei-ed bank at a suitable depth would soon reach 

 the surface. 



The facts relating to the solution of carbonate of lime in 

 sea water are well known and require no recapitulation. Gen- 

 erally, the bottom within the lagoons of the larger atolls and 

 banks was covered between the surface and 1() fathoms with 

 corals, between 10 and 20 fathoms with rubble, and between 

 20 and 25 fathoms with coarse sand passing to fine sand and 

 finally mud at about 40 fathoms. Most of the lagoons are 

 amply provided with passages through their rim reefs, the 

 depths in which seem to bear some relation to the depths within 

 the lagoons, and there is in all an ample circulation of water. 

 Coral sand from the island shores or such as collects on the 

 surface reefs in hollows, contains about '03 per cent of silica, 

 rock on the land which has suffered rain water denudation '06 

 per cent, whilst mud from Suvadiva lagoon 40 to 50 fathoms 

 has 2*4 per cent. The difference is remarkable. Fourteen 

 thoroughly representative surface sands and rocks gave an 

 average of '047 per cent of silica, and muds from three differ- 

 ent parts of Suvadiva lagoon all over 40 fathoms gave 2"441 

 per cent, or fifty times as much. Some of this enormous dif- 

 ference no doubt may be due to Radiolaria and sponge spicules 

 collecting rather in the lagoon, but no explanation seems to nte 

 to be thoroughly adequate that does not consider solution as 

 well. 



Other important factors enter into and undoubtedly assist in 

 the formation of the lagoons, or at the least pi-event them from 

 being filled up. Even within their areas an enormous bulk of 



