150 



DYNAMICAL GEOLOGY. 



149-152. 



■^% 



150 



152 



Islands of the Fiji group: Fig. 149, Goro; 

 150, Angau; 151, Exploring Isles; 152,N'u- 

 muku. 



reef, t, t', alone at the surface, around a lagoon, III, with an islet, xi, over the 

 peak T, wliicli was the last point to disappear. 



These steps are well illustrated at the Fijis. The island Goro (Fig. 

 149) lias a fringing reef 5 Angau (Fig. 150), a barrier; Exploring Isles 

 (Fig. 151), a very distant barrier, with a few islets; Numuku (Fig. 152), 

 a lake with a single rock. The disappearance of this last rock would make 

 the island a true atoll. 



Whenever the subsidence ceases, the waves build up the land above the 

 reach of the tides ; seeds take root ; and the reef becomes covered with 



foliage. 



The lands inside of coral barriers, as 

 illustrated in these figures, very often show, 

 by their narrow broken features and the 

 deep indentations that were once valleys, 

 that they are sunken lands, and thus sus- 

 tain Darwin's theory. 



The atoll Menchikoff (Fig. 145) was 

 evidently formed, as explained by Darwin, 

 about a high island, consisting of two dis- 

 tinct ridges or clusters of summits, like 

 Maui and Oahu in the Hawaiian group. 



If the subsidence be still continued, after the formation of the atoll, 

 the coral island will gradually diminish its diameter, until finally it may 

 be reduced to a mere sand-bank, or become submerged in the depths of 

 the ocean. The occurrence of sunken atolls, like the Maldives, is one of the 

 strong arguments for the theory of subsidence. 



Thickness of reefs. — The thickness of a coral formation, supposing Dar- 

 win's theory to be the true one, is often very great. From soundings within 

 a short distance of coral islands, it is certain that this thickness is in some 

 cases thousands of feet. The barrier reefs remote from an island stand in 

 deep water, approximately proportional in depth to the distance from the 

 coast-line. Supposing the slope of the bottom at the Gambler Islands to be 

 only five degrees, we find, by a simple calculation, that the reef has a thick- 

 ness of 1200 feet. In a similar manner, it is found that the thickness must 

 be at least 250 feet at Tahiti, and 2000 or 3000 at the Fijis. 



The rate of subsidence requh-ed to produce the results described cannot exceed the 

 rate of upward increase of the reef-ground. On page 385 some facts are given illustrat- 

 ing the exceeding slowness of such movements.^ 



As coral debris is distributed, by the waves and currents, according to the same laws 

 that govern the deposition of silt on sea coasts, it does not necessarily follow that the 



1 For further information on the subject of Coral reefs and limestones, the reader may refer 

 to the author's work on Corah and Coral Islands, 400 pp. 8vo., 1891, based on his Exploring 

 Expedition Report on Zoophytes (740 pp. 4to, and 61 plates in folio, 1846) , and to the chapter on 

 Coral Reefs and Islands in his Expedition Report on Geology (750 pp. 4to, with 21 plates in folio, 

 1849) ; also to Darwin on the Structure and Distrihution of Coral Reefs, 8vo, with maps and 

 illustrations, London, 1842, the last edition, by Professor T. G. Bonney, in 1889. 



