192 CORALS AND CORAL ISLANDS. 



The Great Chagos Bank. This bank lies about ten degrees 

 south of the Maldives, and is ninety miles long and seventy in 

 its greatest breadth. It is a part of the Chagos Group, in which 

 there are some true atolls, some bare atoll-reefs, and others, like 

 the Great Chagos Bank, that are quite submerged, or nearly 

 so. Its rim is mostly from four to ten fathoms under water. 



Mr. Darwin confirms the opinion of Captain Moresby, that 

 this bank has the character of a lagoon reef, resemblino- one 

 of the Maldives ; and he states, on the evidence of extensive 

 soundings, that, if raised to the surface, it would actually be- 

 come a coral island, with a lagoon forty fathoms deep. He 

 says that, in the words of Captain Moresby, it is in truth 

 "nothing more than a half-drowned atoll." 



The form of the bank, its margin of shoals, and a line 

 of soundings across it, giving the depth of the central or 

 lagoon portion, are shown in the map on p. 191, from Darwin, 

 and for which, as well as for other information about the 



S Level of llie Sea.. 



76 miles in length- 



EAST AND WEST SECTION ACROSS THE GREAT CHAGOS BANK. 



bank, he gives credit to Captain Moresby. The cross section 

 is still further illustrated in the annexed cut. The whole 

 length of the section (or width of the bank in the line of the 

 soundings) is seventy-six miles. From the outer rim of 

 the submerged atoll, there is a drop off to a deeper level, which 

 is mostly fifteen to eighteen fathoms below the surface ; and 



