348 



MEXICO, CENTEAL AMERICA, WEST INDIES. 



parts of the Gulf and of the Caribbean Sea are derived chiefly from the remains of 

 pteropods, while mineral formations prevail round the seaboard. Silicious sands 

 also cover the beach in some places, and coralline muds or calcareous formations 

 surround the reefs and continue far seawards several peninsulas, amongst others 

 those of Yucatan and Florida. 



The coral builders are at work over a vast range, which may be estimated at 

 one-fourth of the marine surface ; to their incessant toil must be attributed the 



Fij?. 164. 



-Anegada and the Hoeseshoe Reef. 

 Scale 1 : 380,000. 



64-25- 



West or Greenwich 



64'iO- 



Oto 16 

 Feet. 



Depths. 



16 to 80 

 Feet. 



80 to 320 

 Feet. 



320 Feet 

 and upwards 



6 Miles 



formation of those calcareous plateaux by which the straits are contracted on both 

 sides, as well as of those rocky ledges which are washed by high tides, and which 

 are revealed only by sandy dunes, such as the Salt Cay, or by their fringe of 

 mangroves, such as the Anegada, and its prolongation — the dreaded Horseshoe 

 reef — connecting it with the Virgin Islands. More than half of the Cuban sea- 

 board, the various groups of the Bahamas, the eastern members of the Lesser 

 Antilles, and the Bermudas are all of coralline origin. 



