Sect. II. ATOLLS. 41 



slightly inclined ledges or steps : thus at Matilda 

 atoll, 1 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 

 frequently they are quite irregular — some with per- 

 pendicular, some with sloping sides — some rising 

 to the surface, and others lying at all intermediate 

 depths from the bottom upwards. I cannot, there- 

 fore, suppose that the union of such reefs could pro- 

 duce even one uniformly sloping ledge, and much 

 less two or three one beneath the other, and each ter- 

 minated by an abrupt wall. At Matilda Island, which 

 • offers the best example of the step-like structure. 

 Captain Beechey observes that the coral knolls within 

 the lagoon are quite irregular in their height. We 



1 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 perpendicular. 

 This bank is formed of coral and dead shells. 



