AGASSIZ : THE GREAT BARRIER REEF OF AUSTRALIA. 105 



the lighthouse on Breaksea Spit, and from the information gathered 

 by the keeper, as well as from the character of the soundings upon 

 the bank, indicating the former extension of Breaksea Spit, it is evident 

 that the corals upon the spit are dead, and that they probably have 

 been killed by the encroachment of the silicious sands creeping north- 

 ward on the surface of the spit, and from which have been thrown up 

 the fine sand dunes over 300 feet in height which exist on the northern 

 part of the spit. 



This silicious sand is constantly overwhelming the few corals which 

 manage to get established on the spit, and prevent their further growth, 

 much as the silicious sands of Cape Florida are constantly mixing with 

 the calcareous sand derived from the coral reefs immediately to the 

 south of it. 



Owing to the heavy sea running, we were unable to land at Lady 

 Elliot Islet or to examine the pseudo atolls of the Bunker and Cap- 

 ricorn Island groups. Judging however of their structure from the 

 charts and from the excellent description of some of them given by 

 Jukes and the illustrations of Lady Elliot Islet by Kent, and from 

 what I saw subsequently of reef flats which must have been very 

 similar to those of Elliot Islet and of similar islands of the adjacent 

 groups, 1 I imagine that causes very similar to those which have dug out 

 miniature atolls at the Bermudas have acted on a larger scale at these 

 islands. First, the sea wearing away the islets, which may have risen 

 to a considerable height, to the level of the sea, forming a flat on the 

 weather side upon which the swell gradually digs out a shallow lagoon, 

 and on the outer edge of which corals thrive, growing more luxuri- 

 antly on the outer and windward edges. The many shaped coral islets, 

 either with 2 or without 3 remaining pinnacles, attest to their former 

 and greater extent. In some cases all trace of the underlying rocky 

 platform has disappeared, and it has become coated with corals. 



North of Swain Reefs and south of Flinders Passage, the Barrier Reef 

 has been reconnoitred by Captain Flinders, who sketched in a general 

 way the inner limit of the reef patches, and something of the condi- 

 tion of that part of the Great Barrier Reef is known from the track of 



1 Lady Elliot Islet is the most southern of the coral reefs of the Great Bar- 

 rier Reef. It rises some eight or ten feet above high-water mark. Jukes 

 has figured the general aspect of the island (Voyage of the "Fly," Vol. I.), and 

 Kent lias given a fine view of the coral reef flat (Great Barrier Reef, Plate 

 XXXIII. B). 



2 Lady Musgrave, One Tree, Heron, Masthead, Wreck, and N. W. Islands. 



3 Boult, Llewellyn, Fitzroy, Wistari, and Polmaise Reefs. 



