STRUCTURE OF CORAL ISLANDS. 33 



ties were not favorable for a thorough exploration. It has been 

 stated that the more exposed points towards the trades, especially 

 the northeast and southwest, are commonly a little higher than 

 other parts ; and it is altogether probable that some of the sand 

 heaps, there formed, will prove on examination to afford exam- 

 ples of this variety of coral-rock. Such situations are exactly 

 identical with those on Oahu, where they occur on so remarkable 

 a scale. Mr. R. H. Schomburgh states that on the island of 

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

 shores are forty feet in height.* 



Although in these descriptions of atolls, we have dwelt on 

 some points more at length than when describing barrier reefs, 

 still it will be observed that the former have no essential pecul- 

 iarities of structure apart from such as necessarily arise from the 

 absence of high rocky lands. The encircling atoll-reef, corres- 

 ponds with the outer reefs that enclose high islands ; and the 

 green islets with the beach formations, in the two cases, originate 

 in the same manner. 



The lagoons, moreover, are similar in character and position to 

 the inner channels within barrier reefs ; they receive only coral 

 material from the action of degrading agents, because 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 under the lee of a barrier. 

 The corals grow but little disturbed by the waves, and the reef- 

 rock thus formed, often contains them in their natural positions. 



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 Group, 

 the shore platform is seldom as extensive as at the Paumotus. It 

 rarely exceeds fifty yards in width, and is cut up by passages 

 often reaching almost to the beach. It was not unusual for our 

 boats to obtain a landing by watching for a favorable opportunity 

 at the entrance of one of these channels to mount a wave and 

 ride in on its top. In some places the platform is broken into 

 islets. Enderby's Island is one of the number to which this de- 

 scription applies: the beach is eleven or twelve feet high. For 

 the first eight feet, it slopes very regularly at an angle of 30 to 

 35 degrees, and consists of sand, coarse pebbles, or rounded stones 

 of coral, with some shells ; and there is the usual beach con- 

 glomerate near the water's edge. After this first slope, it is hori- 



* Journal of the Royal Geographical Society, ii, 152. Mr. Schomburgh describes 

 the sandhills as 40 feet in height, and behind the first range, a second, and even a 

 third. 



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