Ch. V. OF COKAL-EEEFS. 147 



changes in the state of the surrounding sea or from the 

 subsidence being great or sudden, these Chagos banks 

 present no difficulty. So far, indeed, are any of the above- 

 mentioned cases of dead submerged reefs from offering 

 any difficulty, that their occurrence might have been 

 anticipated on our theory ; and as fresh atolls are sup- 

 posed to be in progressive formation by the subsidence 

 of encircling barrier-reefs, a weighty objection might 

 even have been raised, namely that atolls must increase 

 indefinitely in number, unless proofs of their occasional 

 destruction could have been adduced. 



The Great Chagos Bank. — I have already shown 

 that the submerged condition of the Great Chagos 

 bank (Plate II. fig. 1, with its section, fig. 2) and of 

 some other banks in the Chagos group, may in all pro- 

 bability be attributed to the corals having perished 

 during an unusually rapid or sudden subsidence. The 

 external rim or upper ledge (shaded in the chart) con- 

 sists of dead coral-rock thinly covered with sand; it 

 lies at an average depth of between o and 8 fathoms, 

 and perfectly resembles in form the annular reef of an 

 atoll. The banks of the second level, the boundaries 

 of which are marked by dotted lines in the chart, lie 

 from about 15 to 20 fathoms beneath the surface ; 

 they are several miles in breadth, and terminate in a 

 very steep slope round the central expanse. This 

 central expanse consists of a level muddy flat between 

 30 and 40 fathoms deep. The banks of the second 

 level appear at first sight to resemble the internal step- 



i 2 



