CORAL-REEFS. 95 



their external rim being about eight fathoms submerged, 

 and in being formed of dead rock, with very little living 

 coral on it : a portion nine miles long of the annular reef of 

 Peros Banhos atoll is in the same condition. These facts, 

 as will hereafter be shown, render it very probable that the 

 whole group at some former period subsided seven or eight 

 fathoms ; and that the coral perished on the outer margin of 

 those atolls which are now submerged, but that it continued 

 alive, and grew up to the surface on those which are now 

 perfect. If these atolls did subside, and if from the sud- 

 denness of the movement or from any other cause, those 

 corals which are better adapted to live at a certain depth 

 than at the surface, once got possession of the knolls, 

 supplanting the former occupants, they would exert little or 

 no tendency to grow upwards. To illustrate this, I may 

 observe, that if the corals of the upper zone on the outer 

 edge of Keeling atoll were to perish, it is improbable that 

 those of the lower zone would grow to the surface, and thus 

 become exposed to conditions for which they do not appear 

 to be adapted. The conjecture, that the corals on the 

 submerged knolls within the Chagos atolls have analogous 

 habits with those of the lower zone outside Keeling atoll, 

 receives some support from a remark by Captain Moresby, 

 namely, that they have a different appearance from those on 

 the reefs in the Maldiva atolls, which, as we have seen, all 

 rise to the surface : he compares the kind of difference to 

 that of the vegetation under different climates. I have 

 entered at considerable length into this case, although 

 unable to throw much light on it, in order to show that an 

 equal tendency to upward growth ought not to be attributed 

 to all coral-reefs, — to those situated at different depths, — to 

 those forming the ring of an atoll or those on the knolls 

 within a lagoon, — to those in one area and those in another. 



