624 DYNAMICAL GEOLOGY. 



atoll, the coral island will gradually diminish its diameter, until 

 finally it may be reduced to a mere sand-bank or become sub- 

 merged in the depths of the ocean. 



The rate of subsidence required to produce these results cannot exceed the 

 rate of upward increase of the reef-ground. On page 591 some estimates are 

 given with regard to the exceeding slowness of the movement.* 



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 necessa- 

 rily follow that the existence of a reef in the form of a barrier is evidence of sub- 

 sidence in that region. On page 662 the existence of sand-barriers of similar 

 position is shown to be a common feature of coasts like that of eastern North 

 America. In the cases of the barriers about the islands of the Pacific, however, 

 there is no question on this point. Such barriers do not form about so small 

 islands. Moreover, the great distances of the reefs from the shores, in many 

 cases, and the existence of islands representing all the steps between that with 

 a fringing reef and the true atoll, leave no room for doubt. The remoteness 

 of the Australian barrier from the continent, and the great depth of water in 

 the wide channel, show that this reef is unquestionable proof of a subsidence, — ■ 

 though it is not easy to determine the amount. Along the shores of continents 

 the question whether a barrier coral reef is evidence of subsidence or not must 

 be decided by the facts connected with each special case. (See Appendix E.) 



Recapitulation. — The following are some of the points connected 

 with the formation of limestone strata illustrated by coral reefs : — 



1. The narrow geographical limits of coral-reef rocks at the pre- 

 sent time owing to the existing zones of oceanic temperature. 



2. The narrow limit in depth of the reef-making corals, — it not 

 exceeding 100 feet. 



3. The promiscuous growth of the corals over the reef-grounds. 



4. The perfect compactness and freedom from fossils of a large 

 proportion of the coral rock, although made within a few hundred 

 feet of living corals and shells ; the oolitic structure of part of this 

 compact kind ; while a variety made of broken corals cemented 

 together is common on the seaward side of a reef, and another, 

 made of standing corals with the interstices filled, forms where there 

 is shelter from the ocean's waves. 



5. The aid of the waves of the ocean necessary for making a 



* For further information on the subject of Corals and Coral Islands, the 

 reader may refer to the author's Exploring Expedition Report on Zoophytes, 

 740 pp. 4to and 61 plates in folio, 1846, and to the chapter on the Formation 

 of Coral Reefs and Islands in his Exploring Expedition Geological Report, 

 755 pp. 4to and 21 pi. fol., 1849; also to Darwin on the Structure and Distri- 

 bution of Coral Reefs, 214 pp. 8vo, with maps and illustrations, London, 1842; 

 also to a memoir by Professor Agassiz. 



