CORAL EEEFS. 377 



this volume, which possesses the charm of a simple and 

 perspicuous style, conjoined with great reasoning powers, we 

 shall briefly extract some of these conclusions, as well as the 

 more important facts from which they are derived. 



We have already noticed generally the three classes of 

 atoll, barrier, and fringing reefs, including all the most 

 characteristic varieties of coral formation on the globe. 

 These varieties, however, owing to local or other conditions, 

 are so multiplied in detail, that it would be almost as diffi- 

 cult to give a clear description of them, as to explain the 

 circumstances in which they respectively originate. Without 

 attempting to follow Mr. Darwin in his more ample survey, 

 we may state that he selects as instances of the lagoon islands 

 or atolls, Keeling Island in the Indian Ocean, the vast group 

 of the Maldives, and the extraordinary submerged atoll called 

 the Great Chagos Bank. The first is a single but character- 

 istic specimen of its class. The Maldive Islands form an 

 archipelago of coral atolls, 470 miles in length and about 

 50 miles in breadth ; the atolls ranged in a double line, and 

 some of them of great size as that of Suadiva, 44 miles by 

 34, with an included expanse of water nearly 300 feet deep, 

 and not fewer than 42 channels through which a ship may 

 enter this central lagoon. The Chagos Bank, in the centre of 

 the Indian Ocean, rising abruptly from unfathomable depths 

 to a level near the surface (its longest axis of 90 miles, its 

 breadth from 50 to 70), is well described by Captain Moresby 

 as ' a half-drowned atoll ; ' a view confirmed both by sound- 

 ings, and by the many similar reefs and atolls rising to the 

 surface around it. To this officer we owe admirable surveys 

 of the Maldive and Chagos groups, which have done much 

 to illustrate the subject. 



Of the 'Barrier-reef the most conspicuous instances 

 selected are that great reef, fronting the eastern coast of 

 Australia, with which our readers are now acquainted ; and a 



