Sect. III. GEE AT CHAGOS BANK. 53 



nearly all the rings, moreover, are so perfect, and 

 stand so separate, and the space from which they rise 

 is so level and unlike a true lagoon, that we can easily 

 imagine the conversion of this one great atoll, not into 

 two or three portions, but into a whole group of 

 miniature atolls. A series such as we have here 

 traced, impresses the mind with the idea of actual 

 change ; and it will hereafter be seen, that the theory 

 of subsidence together with the upward growth of the 

 coral-reefs, modified by accidents of probable occur- 

 rence, accounts for the occasional disseverment of large 

 atolls. 



The great Chagos Bank alone remains to be de- 

 scribed. In the Chagos group there are some ordi- 

 nary atolls, some annular reefs rising to the surface 

 but without any islets on them, and some atoll-formed 

 banks either quite or nearly submerged. Of the 

 latter, the Great Chagos Bank is much the largest, and 

 differs in its structure from the others ; a plan of it 

 is given in Plate II. fig. 1, in which, for the sake of 

 clearness, I have had the parts under ten fathoms deep 

 finely shaded : an east and west vertical section is given 

 in fig. 2, in which the vertical scale has been neces- 

 sarily exaggerated. Its longest axis is ninety nautical 

 miles, and another line drawn across the broadest part, 

 at right angles to the first, is seventy. The central 

 part consists of a level muddy flat between forty and 

 fifty fathoms deep, which is surrounded on all sides, 

 with the exception of some breaches, by the steep 



