28 ATOLLS. Ch. I. 



ever, bears a double name), measured in a medial and 

 slightly curved line, is no less than eighty-eight geo- 

 graphical miles long, its greatest width being under 

 twenty, and its least only nine and a half miles. Some 

 atolls have spurs projecting from them ; and in the 

 Marshall group there are atolls united together by 

 linear reefs, for instance Menchioff Island (See Fig. 3, 

 Plate II.), which is sixty miles in length, and consists 

 of three loops tied together. In far the greater num- 

 ber of cases an atoll consists of a simple elongated ring, 

 with its outline moderately regular. 



The average width of the annular reef may be taken 

 at about a quarter of a mile. Capt. Beechey l says 

 that in the atolls of the Low Archipelago it exceeded 

 in no instance half a mile. The description given of 

 the structure and proportional dimensions of the reef 

 and islands of Keeling atoll, appears to apply perfectly 

 to nearly all the atolls in the Pacific and Indian 

 oceans. The islets are first formed some way back 

 either on the projecting points of the reef, especially if 

 its form be angular, or on the sides of the main en- 

 trances into the lagoon — that is in both cases, on points 

 where the breakers can act during gales of wind in 

 somewhat different directions, so that the matter 

 thrown up from one side may accumulate against that 

 before thrown up from another. In Lutke's chart of 

 the Caroline atolls, we see many instances of the former 

 case ; and the occurrence of islets, as if placed for 

 beacons, on the points where there is a gateway or 



1 Beechey's Voyage to the Pacific and Beering's Straits, chap. viii. 



