ECOLOGY OF THE MURRAY ISLAND CORAL REEF. 9 



From the middle of the southeast and along the east and northeast sides 

 of Maer Island the reef forms a wide submerged platform covered by a depth 

 of less than 2 feet at low tide, while along its seaward edge there is a "litho- 

 thamnion ridge," about 150 feet wide and elevated 6 to 8 inches above the 

 low-tide level of spring tide. We will speak later of the structure of this 

 so-called "lithothamnion ridge"; at present it suffices to say that it serves 

 as a dam to impound the water of the great shallow southeast reef-flat at low 

 tide, thus forming a veritable tidal lake about 1,600 feet wide, 8 to 17 inches 

 deep, and more than 2 miles long, from which the water can not escape even 

 though the tide sinks to a lower level on the ocean beyond the reef. A rich 

 growth of corals is found within this marine basin, protected as it is from 

 breakers by the lithothamnion ridge and from the disturbing influences of 

 strong currents by its wide expanse of shallow water. 



No heavy breakers appear to reach the southeast shore at any time, for 

 even during storms the sea expends its force upon the crest of the lithotham- 

 nion ridge. Thus only small shells and pebbles are at present cast ashore 

 along the southeastern side of the island; yet this whole shore-line is bestrewn 

 with large, rounded, beach-worn boulders of black lava which were driven 

 shoreward, rolled in the surf and stranded before the time when the fringing 

 reef grew seaward and caused the breakers to expend their force far out from 

 the original shore-line. (See plate 5 a.) In common with other islands of the 

 Torres Straits region, whether volcanic, calcareous, or continental in charac- 

 ter, the Murray Islands exhibit a recently emerged shore platform about 3 

 feet above the present high-tide level, 1 and wave-worn lava boulders are now 

 found strewn above high-tide level along the entire southeast side of the 

 island and between Hedley Brook and Haddon Brook. (See plate 4 b.) 



The remarkable richness of the southeast reef is probably due in great 

 measure to the pure ocean water which the southeast trade-wind drives con- 

 stantly upon it, thus bringing an abundance of pelagic life to provide food for 

 the corals, cooling and aerating the element in which they live, and enabling 

 the corals to free themselves from silt through the beneficent effect of agitation. 



In contrast with the rich southeast reef-flat, those of the southern and 

 northern ends of the island are relatively devoid of corals, the flats here being 

 covered with drifting silt and sand, which is fatal to madreporean coral 

 growth, although not unfavorable to alcyonaria, such as Sarcophyton. The 

 precipitous outer edges of these reefs are, however, remarkably rich and the 

 largest individual coral heads to be found around the entire island are those 

 at or near the outer edge of the reef-flat of the southern and southwestern 



sides of the island. „ , 



At the northern end of the island the drifting sdt forms a small oval 



sand-bar, about 1,640 feet from shore, 150 feet wide, and elevated about 2 



feet above low-tide level. (See plat e 2.) This sand-bar is composed of fine 



•At V.v,en Point, Thursday Island, large heads of Symphyllia and M*andra : were found by the author 

 ,„ situ in the limestone in which they grew, but they are now emerged about 3 feet above the highest t.des. 



