242 



BULLETIN 103. UNITED STATES NATIONAL MUSEUM. 



upward at such a rate that its top remains near the surface of the 

 water and through retreat of the shore it is converted into a barrier. 

 Continued subsidence, where the inclosed land area is an inland, 

 may result in the production of an atoll circumscribing a lagoon 

 without any land mass projecting above the water level. But this 

 is not all. The Darwinian hypothesis involves more than mere 



Fig. 4. Copy of Darwin's figure illustrating conversion of a fringing into a barrier reef 

 according to his hypothesis. a a — outer edge of the reef at the level of the sea. bb— 

 Shores of the island. A 'A '—Outer edge of the reef, after its upward growth during a 



PERIOD OF SUBSIDENCE. CO— THE LAGOON-CHANNEL BETWEEN THE REEF AND THE SHORES OF THE 

 NOW ENCIRCLED LAND. B'B'— THE SHORES OF THE ENCIRCLED ISLAND. N. B.— IN THIS, AND THE 

 FOLLOWING CUT, THE SUBSIDENCE OF THE LAND COULD ONLY BE REPRESENTED BY AN APPARENT RISE 

 IN THE LEVEL OF THE SEA. 



subsidence and the conversion of a fringing into a barrier reef. It 

 also attempts to account for extensive submarine platforms by 

 assuming that they have been built upon sloping basements through 

 agencies dependent on the presence of reefs. (See text-figs. 4, 5, 6.) 

 Dana's interpretation 1 is essentially that of Darwin. 



i .._ a"., *%__<?_' -41 



Fig. 5. Copy of Darwin's figure illustrating conversion of a barrier reef into an atoll, ac. 

 cording to his hypothesis. a' a 1 — outer edges of the barrier-reef at the level of the sea. 

 the cocoa-nut trees represent coral-islets formed on the reef. cc— the lagoon-channel. 

 B'B'— The shores of the island, generally formed of low alluvial land and of coral detri- 

 tus FROM THE LAGOON CHANNEL. A" A"— THE OUTER EDGES OF THE REEF, NOW FORMING AN ATOLL. 



C— The lagoon of the newly formed atoll. According to the scale the depth of the lagoon 



AND OF THE LAGOON CHANNEL IS EXAGGERATED. 



That Darwin considered an alternative hypothesis is shown by the 

 following quotation : 



I may here observe that a bank either of rock or of hardened sediment, level with 

 the surface of the sea and fringed with living coral, would be immediately converted 

 into an atoll, without passing, as in the case of a reef fringing the shore of an island, 

 through the intermediate form of a barrier reef. 



1 Corals and coral islands, ed. 3, figs. pp. 263, 267, 1890. 



