8 PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



The richness of the Murray Island reefs is due, however, not wholly to 

 their freedom from the destructive effects of severe storms, but to the purity 

 of the deep blue ocean water which lies close to the eastward of them; for 

 here, near the outer edge of the Great Barrier Reef, we find little of the 

 suspended silt which seriously interferes with coral growth around the 

 off-lying islands of Cape York, Australia; nor is there any trace of the muddy 

 shore-water of New Guinea which keeps open the wide Bligh Entrance 

 lying off the great swamps of the Fly River region. 



In common with other islands of Torres Straits, the recent reef-flats 

 surrounding the Murray Islands are much wider on the southeast than on 

 the northwest side of the islands, the reefs having grown mostly to the wind- 

 ward. The strong southeast trade-wind, which prevails for about eight 

 months of the year, causes the ocean water on the incoming tide to sheer near 

 the middle of the southeast side of Maer Island, the currents parting, the 

 stronger going around the southwestern and the weaker around the north- 

 eastern end of the island. The current around the southwestern side is 

 reinforced by that around Dowar Island and is thus stronger than that 

 around the northeastern end. The silt from Haddon and Hedley brooks is 

 thus carried around the southwestern end of the island and contributes to 

 form the sand dunes which are about 20 feet high and to partially cover and 

 smother the reef-flats at the western corner of Maer Island. (See map, plate 2 ; 

 and plate 5 b.) Several smaller sand dunes on the northern corner of the 

 island are also formed by the weaker northwesterly currents and thus the 

 northwest side of the island is concave and lined throughout by a sand-beach 

 formed of volcanic and calcareous fragments. It is interesting to observe that 

 the sand derived from these currents is tending to change the original oval 

 shape of the island into a crescent, reminding one of the manner in which an 

 atoll islet acquires its typical crescentic shape, as shown by Guppy,' Hedley 

 and Taylor, Wood Jones, and Vaughan. The outflowing currents due to 

 the falling tide are not competent to offset this effect, for they must make 

 their way against the prevailing southeast wind. At the Murray Islands the 

 tide rises between 7 and 8 feet, thus producing spring tide currents of nearly 

 4 knots an hour around the southern end and a flow of about half that rate 

 around the northern end of the island. 



Measured from the shore-line at mean high tide, the reef-flat from the 

 middle of the southeast side of the island to the northern corner is from 1,800 

 to 2,200 feet in width. At the middle of the northwest side it is 780 feet 

 wide, while off the sand dunes at the western corner the outer edge of the 

 reef is only 175 feet from shore, about 85 feet of this distance being submerged 

 sand-beach, which is laid bare at low tide, thus leaving only about 90 feet 

 of Sarcophyton and other alcyonaria and coral-bearing reef-flat. Around 

 the southwestern end of the island the reef-flat is about 400 to 600 feet in 

 width. 



'Guppy, Scottish Geographical Magazine, vol. j, pp. 472-474, 1889. 



