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BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



with the shallowest part of the reef. This belt of heads rarely extends 

 to a greater depth than one and a half to two fathoms, and has a width 

 of usually less than one hundred yards, thus leaving the whole interior 

 of the lagoon free from coral heads. Judging from the descriptions of 

 Dana, Darwin, and others of the growth of coral heads in the interior 

 of other lagoons, their absence in a lagoon as open as that of Hogsty 

 Reef seems to be an exception. This is the more remarkable as the 

 position of this atoll is such that from its constant exposure to the 

 action of the prevailing trades a great mass of fresh sea water is con- 

 stantly poured into the lagoon for more than half its circumference. 

 This mass of water of course washes constantly the scattered heads par- 

 allel to the line of least depth, but also throws into the inner lagoon 

 water loaded with particles of sand, which cover its floor and leave ap- 

 parently no chance for the growth of anything except algaj and corallines 

 in the main basin of the lagoon. It is quite evident, when watching the 





•v^^ 





HOGSTY KEEF. 



huge trade wind breakers following one another in rapid succession, that 

 they must act as a very efficient force pump in driving an immense quan- 

 tity of water to the westward, thus constantly changing the water in 

 the lagoon, and rapidly removing the carbonate of lime it liolds both in 

 solution and in suspension.^ It is only when we reach the western part 

 of the atoll, where the depth increases, that a great part of the water 

 forced into the lagoon by the breakers finds its way out in part through 

 passages both on the north and south faces, but mainly through the 

 wide western entrance of the lagoon. The entrance is nearly a mile 

 wide between tlie 3 fathom lines of opposite sides, and the section 

 across it sliows a greater depth (five, six, and seven fathoms) than at 

 any other section of the lagoon. A strong westerly current is always 

 running out, probably due to the mass of water incessantly piled up 

 by the breakers into the eastern half of the comparatively closed por- 

 tion of the lagoon. 



1 Murray and Irvine, Proc. Roy. Soc. of Edinburgli, 1889, p. 79. 



