26 CORAL-REEFS. 



and when alive is of a beautiful pale lake-red : a Madre- 

 pora, closely allied or identical with M. pociillfera, is also 

 common. As soon as an islet is formed, and the waves are 

 prevented breaking entirely over the reef, the channels and 

 hollows in it become filled up with cemented fragments, 

 and its surface is converted into a hard smooth floor (C of 

 woodcut), like an artificial one of freestone. This flat sur- 

 face varies in width from ioo to 200, or even 300 yards, 

 and is strewed with a few large fragments of coral torn up 

 during gales : it is uncovered only at low water. I could 

 with difficulty, and only by the aid of a chisel, procure 

 chips of rock from its surface, and therefore could not 

 ascertain how much of it is formed by the aggregation of 

 detritus, and how much by the outward growth of mounds 

 of corals, similar to those now living on the margin. 

 Nothing can be more singular than the appearance at low 

 tide of this 'flat' of naked stone, especially where it is 

 externally bounded by the smooth convex mound of 

 Nulliporae, appearing like a breakwater built to resist the 

 waves, which are constantly throwing over it sheets of 

 foaming water. The characteristic appearance of this 

 'flat' is shown in the foregoing woodcut of Whitsunday 

 atoll. 



The islets on the reef are first formed between 200 and 

 300 yards from its outer edge, through the accumulation of 

 a pile of fragments, thrown together by some unusually 

 strong gale. Their ordinary width is under a quarter of a 

 mile, and their length varies from a few yards to several 

 miles. Those on the S.E. and windward side of the atoll, 

 increase solely by the addition of fragments on their outer 

 side j hence the loose blocks of coral, of which their sur- 

 face is composed, as well as the shells mingled with them, 

 almost exclusively consist of those kinds which live on the 



