CORAL-REEFS. 33 



supposition which I must not pass over. If the whole, or 

 a large part of the 'flat,' had been formed by the outward 

 growth of the margin, each successive margin would 

 naturally have been coated by the Nulliporae, and so much 

 of the surface would have been of equal height with the 

 existing zone of living Nulliporse : this is not the case, as 

 may be seen in the woodcut. It is, however, evident from 

 the abraded state of the ' flat,' with its original inequalities 

 filled up, that its surface has been much modified ; and it is 

 possible that the hinder portions of the zone of Nulliporse, 

 perishing as the reef grows outwards, 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, the increase of the atoll in its 

 different parts : there are others having an opposite tend- 

 ency. On the S.E. side, Lieut. Sulivan, to whose kindness 

 I am indebted for many interesting observations, found the 

 conglomerate projecting on the reef nearly fifty yards in 

 front of the beach : we may infer from what we see in all 

 other parts of the atoll, 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 

 fragments cast on the beach, is certainly very surprising, 

 even on the view that it is the office of occasional gales to 

 pile up fragments, and of the daily tides to wear them 



868 



