Sect. II. 
ATOLLS. 
41 
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 at Vanikoro have 
a similar structure. Captain Beechey has described a 
modification of this structure (and he believes it is not 
uncommon) 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 inwards, ends abruptly in a little 
submarine cliff three fathoms deep ; at its foot, a 
ledge 40 yards in width also shelves gently in- 
wards, like the surface-reef, and terminates in a 
second little cliff five fathoms deep ; beyond this, the 
bottom of the lagoon slopes to 20 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 
1 Kotzebue’s First Voyage, vol. iii. p. 142. 
2 Beeehey’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. 
