Coral Reefs and Islands. 283 



tions of reefs increase faster than the inner, owing to the 

 purer water about them and the more abundant life for food ; 

 that the inner parts are not only at a disadvantage in these 

 respects, but suffer also from coral debris thrown over them. 

 They add to these causes of unequal growth mentioned by 

 Darwin, the solvent and abrading action of the waters. 



It is hence concluded that, under these conditions, the 

 simple bank of growing corals may have a depression made at 

 the centre, which, as the process continues, will become a lagoon 

 basin, and the reef thereby an atoll with a shallow lagoon ; 

 that the atoll, so begun, may continue to enlarge through the 

 external widening of the reef and the further action of 

 current-abrasion and solution within : or, in the case of 

 fringing reefs, that the change may go on until the reef has 

 become a barrier-reef, with an inner channel and inner reefs. 

 It is admitted that subsidence may possibly have helped in 

 the case of the deepest lagoons. 



Dr. Greikie expresses his opinion on the subject thus : — " As 

 the atoll increases in size the lagoon becomes proportionally 

 larger, partly from its waters being less supplied with pelagic 

 food, and therefore less favourable to the growth of the more 

 massive kinds of corals, partly from the injurious effects of 

 calcareous sediment upon coral-growth there, and partly also 

 from the solvent action of the carbonic acid of the sea-water 

 upon the dead coral." 



Mr. Semper gives examples of the effects of currents at the 

 Pelew Islands, stating that, by striking against or flowing by 

 the living corals, they make the reef grow with steeper sides 

 and determine its direction, and urging that abrasion and 

 solution have made, not only the deep lagoon-like channels, 

 but the deeper channels between the islands. He holds that 

 in Kriangle, which he describes as a true atoll with no 

 channel leading into the lagoon from the sea, the lagoon 

 may have been a the result of the action of currents on the 

 porous soil during a period of slow upheaval *. He says, 

 further, that the large channel in the main island of the group 

 " forty fathoms deep and many miles wide," " finds an easy 

 explanation on the assumption of an upheaval ;" it became 

 " wider in proportion as the enclosed island, consisting of soft 

 stone [tufa], was gradually eaten away ; and during slow 

 upheaval it would continue to grow deeper in proportion as 

 the old porous portions of the reef and the rock in which it 

 was forming were more and more worn down by the combined 

 action of boring animals and plants, and of the currents 



* ' Animal Life/ pp. 269, 270. 



