1 909.] THEORY OF ATOLL FORMATIOX. 673 



that the lagoon tends to become steadily more shallow, Avhile the 

 actual growth of coral upon its bottom actually diminishes. In 

 some cases, and in certain stages of atoll development, this change 

 takes place rapidly, so that in the human history of the atoll 

 great alterations have taken place in lagoon configuration. Again, 

 the lagoons of many atollons, and of some atolls, become entirely 

 obliterated and dry land joins island to island across the ring. 

 The theory that furnishes an explanation for the origin of these 

 structures must not be in opposition to this known fact of their 

 after history. Some high oceanic islands possess a reef that 

 surrounds their coast-line and is a mere outskirt to their land ; 

 while, in some cases, the reef stands out from, the shore, and a 

 channel of shallow water intervenes between the shore-line and 

 the reef. These features must be accounted for, for the near- 

 shore reef and the outstanding reef are evidently very similar 

 formations to the reef that is seen as the seaward margin of the 

 breccia platform of atolls. 



Finally, colonies of Porites, and other corals of a massive habit 

 of growth, tend, with their increase of size, to become first 

 flattened at the top and then basin-shaped, — only the outer edge 

 of the top of the colony being a raised rim of living coral. This 

 formation is strangely like an atoll in miniature, and its develop- 

 ment must be carefully studied. 



(ii.) The theories put foru:arcl to explain these problems and a 

 discussion of some observed facts that tend to contradict 

 them. 



(a) The Theory of Subsidence ; first brought forward by Darwin 

 in 1837 (Proc. Geol. Soc. vol. ii. p. 552). In this theory the 

 sinking of the land basis was the cause of all the typical features 

 of fully developed coral structures. 



The oceanic bank is assumed to be the remains of old land sunk 

 beneath the waves. The steps of development are as follows. An 

 oceanic island is situated in a sea the conditions of which, such 

 as constant temperature &c., permit the flourishing of the reef- 

 building corals. The coral colonies grow around its shores wherever 

 the submarine slope furnishes foothold within their bathymetrical 

 range. The island becomes surrounded by a reef. The process of 

 subsidence causes the island to sink slowly beneath the waves. The 

 reef continues to grow upwai^ds, especially at its outer edge, for 

 Darwin said that better aeration by waves, and more abundant 

 food, nourished the outer colonies ; there is no compensation for the 

 sinking land, and the island becomes surrounded by a moat, girt 

 about by an outstanding coral-reef. The process goes on : the 

 land finally sinks within, and the reef — upon which islands are 

 afterwards developed — encloses a lagoon in which the original 

 island has disappeared. Finally, when subsidence is too rapid for 

 the upgrowth of the coral-reef to keep pace with it, the whole 

 structure sinks beneath the waves as a reef with raised margins — 

 Proc. Zool. Soc— 1909, No. XL^T!. 46 



