ISO ■ COA'ALS AiVD COITAL ISLAiVDS. 



stone, excepting in its whiter colour ; but generally its sand 

 origin is very apparent. On the northern atolls of the Maldives 

 the beach sand-rock is said to be quarried out in square blocks 

 and used for building. 



In borings by Lieutenant Johnson, of the Wilkes Exploring 

 Expedition, on Aratica or Carlshoff's Island, in the Paumotus, 

 ten or eleven feet were passed through easily, and then there 

 was a sudden transition from this softer rock (probably the beach 

 sand-rock) to the solid reef-rock. 



The d?ift sand-rock was not met with by the author on any 

 of the coral islands visited. The time for exploration on these 

 islands allowed by the Expedition was too short for thorough 

 work. It has been stated that the more exposed points toward 

 the trades, especially the north-eastern and south-western, 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 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. R. H Schomburgh, 

 in an article in the Journal of the Royal Geographical 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, aiid 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 pecuharities of structure apart from such as necessarily 

 arise from the absence of high rocky lands. The incircling 

 atoll reef corresponds with the outer reefs that inclose 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 position 

 to the inner channels* within barrier reefs ; they receive coral 

 material only from the action of degrading agents, because no 

 other source of detritus but the reefs is at hand. The 



