588 



FOKMATION OF COEAL EEEFS. 



[Ch. XLIX. 



me 



In tlie above description tlie solid stone is stated to consist 

 of shell and coral^ united bj sand ; but masses of very compact 

 limestone are also fonnd even in the uppermost and newest 

 parts of the reef, such as could only have been produced by 

 chemical precipitation. Professor Agassiz also informs 

 that his observations on the Florida reefs (which confirm 

 Darwin's theory of atolls to be mentioned in the sequel) 

 have convinced him, that large blocks are loosened, not by 



imagined, but by 

 innumerable perforations of lithodomi and other borincr tes- 



shrinkage in the sun's heat, as Chamisso 



tacea. 



may 



from the decomposition of corals and testacea ; for when the 

 animal matter undergoes putrefaction, the calcareous resi- 

 duum must be set free under circumstances v^^T favourable 

 to precipitation, especially when there are other calcareous 

 substances, such as shells and corals, on which it may be 

 deposited. Thus organic bodies may be enclosed in a solid 

 cement, and become portions of rocky masses."^ 



The width of the circular strip of dead coral forming the 

 islands explored by Captain Beechey, exceeded in no instance 

 half a mile from the usual wash of the sea to the edge of the 

 lagoon, and, in general, was only about three or four hundred 



t 



from 



The two other peculiarities which are most characteristic of 

 the annular reef or atoll is first, that the strip of dead coral 

 is invariably highest on the windward side, and secondly, 

 that there is very generally an opening at some point in the 

 reef affording a narrow passage, often of considerable depth, 

 from the sea into the laoroon. 



Maldive and Laccadive Isles. — The chain of reefs and 

 islets called the Maldives (see fig. 

 Indian Ocean, to the south-west of 



470 geographical miles in length, running due north 

 and south, with an average breadth of about 50 miles. It 



'^ Stutchbury, AVost of Eng. Journ., Joimi. Geol. Soc, Nov. 1864, p. 360. 

 No. i. p. 50, and P. M. Duncan, Quart. f Captain Beecliey, part i. p. 188. 



155 

 Malab 



situated in the 



I 



