STRUCTURE OF CORAL ISLANDS. 185 



on examination to afford examples of this variety of coral- 

 rock. Such situations are exactly identical with those on 

 Oahu, where they occur on so remarkable a scale. Mr. Li. H. 

 Schomburgh, in an article in the Journal of the Royal Geo- 

 graphical Society, vol. ii., p. 152, states that on the island of 

 Anegada, in the West Indies, the drift banks on the windward 

 shores are forty feet in height, and that behind the first range 



there is a second, and even a third. 



» 



Although in these descriptions of atolls, some points have 

 been dwelt upon more at length than in the description of 

 barrier reefs, still it will be observed that the former have no 

 essential peculiarities of structure apart from such as necessar- 

 ily arise from the absence of high rocky lands. The encircling 

 atoll reef corresponds with the outer reefs that enclose high 

 islands ; and the green islands and the beach formations, in the 

 two cases, originate in the same manner. 



The lagoons, moreover, are similar in character and posi- 

 tion to the inner channels within barrier reefs ; they receive 

 coral material only from the action of degrading agents, be- 

 cause no other source of detritus but the reefs is at hand. The 

 accumulations going on within them are, therefore, wholly of 

 coral. The reefs within the lagoons correspond very exactly 

 in mode of growth and other characters to the inner reefs un- 

 der the lee of a barrier. 



IV. NOTICES OF SOME CORAL ISLANDS. 



The preceding descriptions represent the general character 

 of atolls, but are more especially drawn from the Paumotus. 

 There are some peculiarities in other seas, to which we may 

 briefly allude. 



Among the scattered coral islands north of the Samoan 



