Sect. I. KEELING^ ATOLL. 23 



each successive margin would naturally have been 

 coated by the Nulliporse, and so much of the surface 

 would have been of equal height with the existing zone 

 of living JNulliporse : this is not the case, as may be 

 seen in the wood-cut. It is, however, evident from 

 the abraded state of the 6 flat,' with its original ine- 

 qualities filled up, that its surface has been much 

 modified ; and it is possible that the inner portions of 

 the zone of Nulliporae, perishing as the reef grows out^ 

 wards, might be worn down by the surf. If this has 

 not taken place, the reef can in no part have increased 

 outwards in breadth since its formation, or at least 

 since the Nulliporae formed the convex mound on its 

 margin : for the zone thus formed, and which stands 

 between two and three feet above the other parts of 

 the reef, is nowhere much above twenty yards in 

 width. 



Thus far we have considered facts, which indicate, 

 with more or less probability, an increase in the 

 diameter of the atoll ; but there are others having an 

 opposite tendency. On the S.E. side, Lieut. Sulivan, 

 to whose kindness I am indebted for many interesting 

 observations, found the conglomerate (D, in wood cut) 

 projecting on the reef nearly fifty yards in front of 

 the islets : we may infer from what we elsewhere see 

 that the conglomerate was not originally so much 

 exposed, but formed the base of an islet, the front and 

 upper part of which has since been swept away. The 

 degree to which the conglomerate, round nearly the 

 whole atoll, has been scooped, broken up, and the frag- 



