CORAL-REEFS. 37 



Kotzebue) several of the atolls are more than thirty miles 

 in length, and Rimsky Korsacoff is fifty-four long, and 

 twenty wide, at the broadest part of its irregular outline. 

 Most of the atolls in the Maldiva Archipelago are of great 

 size, one of them (which, however, bears a double name) 

 measured in a medial and slightly curved line, is no less 

 than eighty-eight geographical 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 Menchicoff Island (see Fig. 2, Plate I.), 

 which is sixty miles in length, and consists of three loops 

 tied together. In far the greater number of cases an 

 atoll consists of a simple elongated ring, with its outline 

 moderately regular. 



The average width of the annular wreath may be taken 

 as about a quarter of a mile. Capt. Beechey 1 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 islets 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 entrances 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 breach through the 



1 Beechey' s Voyage to the i acific and Beering's Straits t chap. vni. 



