MR GARDINER ON ATOLLS. I 23 



in height. These islets are generally undermined for 10 — 20 feet 

 between tide levels, and have narrow reefs around them, which 

 however extend only to the edges of the overhanging rocks, ending 

 precipitously against the lagoon. These give direct evidence of 

 solution, for of large masses, which can be seen to have split off and 

 fallen into the lagoon only recently, little trace can be found and 

 the reefs formerly under them are often completely gone. Finally 

 Murray and Irvine' have shown that globigerina ooze, coral sand 

 and corals, indeed the ordinary constituents of a modern coral reef, 

 are several times as soluble as such limestone in sea water. 



In further support of Murray's solution theory I may draw 

 attention to the great uniformity in breadth of the reefs of the 

 Pacific, which I think may best be explained on some such hypo- 

 thesis ; and also the small clilT from the edge of the reef to the 

 bottom of the lagoon, which, as far as I have seen, is of very 

 general occurrence, and would not be found were the lagoons being 

 gradually filled up by detritus as the subsidence theory requires. 



The Lau Group suggests another method of the formation of 

 atolls by the complete solution and erosion particularly of raised 

 limestone islands. Although I am inclined to believe that most of 

 the atoll-reefs of Fiji have been formed in this way, yet I do not 

 consider that it is anywhere of more than local importance. 



Lastly, as to the formation of islets on the reefs, I would draw 

 attention to the opinions of Mr G. C. Bourne and Prof Sollas as to 

 the elevations of Diego Garcia and Funafuti atolls. With regard 

 to the latter I entirely agree with Prof Sollas, and further, from 

 the information given me by Mr J. J. Lister as to the Phoenix 

 Group and by numerous trading captains in Fiji as to the Ellice, 

 Union, Gilbert, Marshall and Caroline Groups, I am inclined to 

 believe that all the atolls of the Central Pacific have been so raised, or 

 possibly that there has been some change of level in the sea. It must 

 be noted, however, that Mr Hedley'^ for Funafuti and Dr Guppy for 

 the Cocos-Keeling Atoll consider that the islets have been rather 

 formed by the heaping up of material from the reef in heavy storms 

 and hurricanes. I would ask Mr Hedley to explain why, on 

 Funafuti, corals are found so frequently in the position of growth 

 on masses or " pinnacles " 4 — 5 feet above the low-tide level. I have 

 been informed too by Mr Bourne that he likewise found subfossal 

 corals on Diego Garcia absolutely similarly situated. 



Mr Bourne accentuated the rarity of occurrence of Copepods, 

 &c., in the gastrovascular cavity of the coral anthozoa and the abund- 

 ance of Zooxanthellae. Confirmed congregation of Zooxanthellae 

 in outer layers of colony, i.e. next light. He recommended the 

 study of the coral reefs of the Indian Ocean. 



1 "On Coral Reefs and other Carbonate of Lime Formations in Modern Seas,'' Proc. 

 R. Hoc, Edin., vol. xvn, p. 79 (1889). 



- "The Broadening of Atoll-Islets," Nat. Set., vol. xn, p. 174 (1898). 



