1888.] and the Coral Formations of the Indian Ocean. 



443 



show where the combined action of wind and waves has added a new 

 piece to the island. Within the lagoon the currents are constantly 

 changing in force and direction, and their every change affects the 

 growth of coral in their track. In estimating the structure of the 

 atoll these changes should be kept in mind, although their complexity 

 makes it far more difficult to arrive at a correct conclusion. 



In the course of my investigations I learnt to distinguish the 

 following kinds of coral rock formed by the action of the waves or 

 wind or both combined. 



Firstly, reef rock, a tolerably homogeneous mass of compacted coral 

 debris, the component parts of which are so thoroughly infiltrated 

 with carbonate of lime held in solution in the sea-water that the 

 masses and fragments of coral composing the rock are rarely distin- 

 guished from one another. This form of rock exhibits a fine hori- 

 zontal stratification; it is invariably formed under the sea or between 

 tide marks. 



Secondly, boulder rock, formed just above high tide mark by means 

 of the masses of coral which are transported across the reef by the 

 waves and are piled up to form the low rampart already alluded to. 

 The interstices of the boulders are soon filled up with coral debris and 

 sand, and are cemented together by the spray. Such rock is only 

 formed on the seaward shores and invariably shows a stratification 

 dipping downwards towards the sea. 



Thirdly, shingle rock, which may be of two kinds. The first kind is 

 horizontally stratified and is scarcely distinguishable from reef rock, 

 except in its finer texture ; it is formed below water or between tide 

 marks by the agglomeration of small pieces of broken coral, among 

 which are included numerous shells of molluscs, remains of Crustacea, 

 echinoderms, &c, and in the more sheltered parts of the lagoon it may 

 include considerable masses of dead madrepores imbedded in their 

 natural position in the rock. This rock is of looser texture than the 

 reef rock. The second kind of shingle rock is formed above high- 

 water mark by the action of the waves. It is entirely composed of 

 small fragments and exhibits a fine stratification dipping seawards at 

 an angle. 



Lastly, there is the sand rock formed above water by the action of 

 the wind. Wherever masses of fine sand are piled up within reach of 

 the spray they are gradually compacted, and form a friable rock, the 

 stratification of which dips seaward. 



In many parts of the island I observed that the land was composed 

 of stratified reef or shingle rock, the strata of which were perfectly 

 horizontal, and did not dip down towards either shore. Having 

 observed the manner in which the different kinds of coral rock were 

 formed, I was at a loss to understand how such horizontally stratified 

 masses could have been formed in their present position above high 



