48 CORAL-REEFS. 



where there are coral-reefs ; but this is by no means the 

 universal structure in other atolls. Chamisso, 1 speaking in 

 general terms of the lagoons in the Marshall atolls, says the 

 lead generally sinks " from a depth of two or three fathoms 

 to twenty or twenty-four, and you may pursue a line in 

 which on one side of the boat you may see the bottom, and 

 on the other the azure-blue deep water." The shores of 

 the lagoon-like channel within the barrier-reef of Vanikoro 

 have a similar structure. Captain Beechey has described a 

 modification of this structure (and he believes it is not un- 

 common) in two atolls in the Low Archipelago, in which 

 the shores of the lagoon descend by a few, broad, slightly 

 inclined ledges or steps : thus at Matilda atoll, 2 the great 

 exterior reef, the surface of which is gently inclined 

 towards and beneath the surface of the lagoon, ends 

 abruptly in a little cliff three fathoms deep ; at its foot, a 

 ledge forty yards wide extends, shelving gently inwards 

 like the surface-reef, and terminated by a second little cliff 

 five fathoms deep ; beyond this, the bottom of the lagoon 

 slopes to twenty fathoms, which is the average depth of its 

 centre. These ledges seem to be formed of coral-rock; 

 and Captain Beechey says that the lead often descended 

 several fathoms through holes in them. In some atolls, all 

 the coral-reefs or knolls in the lagoon come to the surface 

 at low water ; in other cases of rarer occurrence, all lie at 

 nearly the same depth beneath it, but most frequently they 

 are quite irregular, — some with perpendicular, some with 

 sloping sides, — some rising to the surface, and others lying 



1 Kotzebue's First Voyage, vol. iii. p. 142. 



2 Beechey's Voyage, 4to ed., vol. i. p. 160. At Whitsunday 

 Island the bottom of the lagoon slopes gradually towards the centre, 

 and then deepens suddenly, the edge of the bank being nearly perpen- 

 dicular. This bank is formed of coral and dead shells. 



