THE CORAL-REEF PROBLEM. 49 



feet ; beyond this depth we meet only with dead coral. In 

 the case of the Bermudas, and in that of a few other reefs, 

 the temperature of the water has been known to descend to 66 

 or even 64 F., but this condition is very exceptional. 



One of the most familiar and wide-spread types of coral 

 structure is the atoll, which acquires special development in 

 the waters of the Pacific and Indian oceans. It consists of a 

 more or less irregular ring of living and dead coral, enclosing 

 within its boundaries an internal body of water (lagoon), which 

 in many cases is kept in direct communication with the ex- 

 terior by one or more connecting channels of water ; the breaks 

 in the ring corresponding-'to these passages almost invariably 

 occur on the leeward side of the island. The atoll, although 

 frequently so described, -is rarely of a circular form, the outline 

 being very generally elongated and angular. In extent it 

 varies from two to three miles, or less, in length to upwards of 

 40 or 50 miles ; where the dimensions are very small the lagoon 

 may be completely absent, or merely indicated by a dry de- 

 pression. The breadth of the coral ring itself dx)es not usually 

 exceed 1000-1500 feet, or somewhat more than a quarter of a 

 mile. In the general composition of an atoll, the following 

 parts may be recognized : first, an outer platform of coral-rock, 

 more or less exposed at low water, which is the correspondent 

 of the ordinary rock platforms resulting from tidal destruction ; 

 secondly, the beach-line proper, measuring a few feet in height, 

 and consisting of coral sand, calcareous pebbles, and triturated 

 shells; and thirdly, the exposed ring itself with the width as 

 above stated, over which, more especially on the windward side, 

 a luxuriant vegetable growth is developed. The elevation of 

 this portion of the atoll more commonly does not exceed 10-20 

 feet, although exceptionally the wind-swept dunes of coral sand 

 attain a much greater height. On some islands, not necessarily 

 atolls, however, as Anegada, one of the West Indies, the drift 

 banks rise to a height of 40 feet, while on the Bermudas they 

 considerably exceed 200 feet, reaching at one point, Sears' 

 Hill, 260 feet. 



