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 fect in height which exist on the northern 
part of the spit. 
This silicious sand is constantly overwhelming the few corals which 
manage to get ostablished on the spit, and prevent their further growth, 
much as the silicious sands of Cape Florida are constantly mixing with 
the calearcous 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 D saw subsequently of reef flats which must have been very 
similar to those of Elliot Islet and of similar islands of the adjacent 
groups,! 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? or without? remaining pinnacles, attest to their former 
and greater extent. Іп 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 sketehed 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.Тайу 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 has given a fine view of the соға! reef flat (Great Barrier Reef, Plate 
XXXIIL В). 
2 Lady Musgrave, One Tree, Heron, Masthead, Wreck, and М. W. Islands. 
3 Boult, Llewellyn, Fitzroy, Wistari, and Polmaise Reefs. 
