170 CORAL AND ATOLLS 



bo gone, and the breccia platform will be laid bare to the rise 

 of coral boulders. 



The nature of the beach depends entirely on the force of 

 the sea which usually beats upon it. Where the waves are 

 normally high, and the shore is exposed to the full force of 

 the Trade-driven swell, the surf will rush over the submerged 

 flats at full tide, and exert its whole energy on the boulders of 

 the beach. It is in such places that the island rise is abrupt 

 and the beach is made up of great masses piled one upon the 

 other. All the sand, and the smaller debris, is washed away, 

 only to be deposited on any part of the shore where a normally 

 calmer state of things prevails. The character of the beach is, 

 therefore, a map, which, when studied aright, gives up its story 

 of the effects of the currents and winds, and the negative or 

 positive movements to which the shore platforms have been 

 in the past subjected ; but the temporary piling of sand or its 

 temporary removal must not be taken as evidences of actual 

 changes of level, or of constant action. On the seaward 

 beaches is thrown the flotsam and jetsam that reaches the 

 group from the outside world, and one of the principal items, 

 — one that has in many places caused a considerable alteration 

 in the character of the islands — is pumice. The greater part of 

 the pumice found in the group arrived after the eruption of 

 Krakatua : being washed up in 1883 in vast quantities. This 

 pumice, lightest of all the wrack that the sea has piled up, has 

 been carried for varying distances into the island from the 

 seaward beach, and shows, as an index, the limit of surf action in 

 the island building that has been reached in twenty-three years. 

 It occurs in great quantities as rounded sea-worn masses, some 

 being a foot and more in their long axis, but the majority 

 varying from the size of marbles to that of cricket balls. 

 Besides the Krakatua pumice, which lies to-day mostly on the 

 seaward beaches, and for a few paces into the island itself, 

 there is older pumice which may be found almost anywhere in 

 the breadth of the dry land. Pumice has been arriving from 

 somewhere ever since the first appearance of land in the atoll 

 ring, and has, during the period of its stay, undergone much 



