I3V C. HEDLEY. 455 



■my account of the mollusca by a description of our collecting 

 ■station. 



Mast Head Reef lies just outside the Tropic of Capricorn; it is 

 the most western, and therefore nearest to the mainland, of a 

 chain of reefs extending for 54 miles from North Reef to Lady 

 Elliott Island. A continuation of the axis along which these 

 reefs are arranged would pass through Beaksea Spit. Strictly 

 speaking, this group is not a part of the Great Barrier, which 

 terminates in a coral maze, Swain Reefs, north of the Capricorns. 

 Between Swain Reefs and the Capricorns lies the broad and deep 

 -Curtis Channel. But for zoological purposes these pseudatolls 

 may conveniently be regarded as a continuation of the Creat 

 Barrier. 



The sketch map on the next page explains the position of the 

 reef cluster. 



At low tide Mast Head is exposed as an oblong reef about f out- 

 miles from east to west and one and a half from north to south, 

 its crest ten or twelve feet above the sea. It shrinks at high- 

 water to a small but densely vegetated sarrlbank about 100 acres 

 in extent, placed near the western end of the reef. The islet is 

 level, raised but a few feet above the sea, and has no lagoonlet. 

 Concentric undulations show the successive increase of beach built 

 on beach. At the western end a clump of uprooted Casuarinas 

 indicates where a gale had inflicted temporary loss. 



On the south side a stratum of coral-sand-rock is now suffering 

 denudation. Jukes has given* an excellent description and 

 explanation of this rock. Though considerable literature has 

 gathered on the subject, this account by Jukes seems not to have 

 atttracted the attention it deserves. As my own observations 

 had independently brought me to the same conclusion, I will 

 restate the case. This rock appears to originate as follows : — 

 Water percolating through a drift of coral sand dissolves lime and 

 carries it down to lower levels. Where the water stops lime is 

 deposited, cements the sand grains and forms an obstruction for 



* Jukes, Voy. ' Fly,' i. 1847, pp.7-9. 



