ECOLOGY OF THE MURRAY ISLAND CORAL REKF. 43 



Scouring by currents and disintegration are, however, of limited efficacy 

 in the deepening of lagoons in the Murray Islands, for the lagoon of the wide 

 southeast reef is only about i8 inches deep at low tide. The effects of 

 scouring and disintegration are, however, constantly counteracted by the 

 thickly-clustered growth of coral heads over the bottom; but on the other hand 

 the corals are being unceasingly disintegrated by wave-action as well as by the 

 activity of echini, holothurians, fish, and other organisms, in the manner 

 described in detail by J. Stanley Gardiner, Wood Jones, Vaughan, Duerden, 

 and others. Thus, independent of solution due to CO2 provided by the 

 decomposition of plant and animal matter, there are factors which tend 

 to deepen the lagoons, while others tend to fill them up, and the resultant 

 condition represents the balance between antagonistic tendencies, and most 

 if not all lagoons are at present being filled up by an accumulation of lime- 

 stone, mud, and silt carried into them in the manner described by Guppy,' 

 wherein he claims that at least 5,000 tons of sand are annually drawn into 

 the lagoon of Cocos-Keeling through openings between the islets. Wood 

 Jones also decides that this lagoon is filling up, and the same phenomena are 

 reported by Hedley and Taylor for the Great Barrier Reef, and by Vaughan 

 for the Florida-Bahama region and the West Indies, and under certain con- 

 ditions the varying solubility of different forms of calcium carbonate may lead 

 to the conversion of coral into limestone in the manner explained by Wells,^ 

 who shows that with fluctuations in temperature particles of calcium car- 

 bonate alternately dissolve in and redeposit in the pore space, tending 

 always toward the more stable calcite. 



On the flats of the fringing reef surrounding the Murray Islands, however, 

 the balance is set slightly in the other direction and there is evidence of the 

 disappearance of a layer of limestone over the southeast reef, at least 1,200 

 feet wide and about 2 feet thick; for the amount of loose sand overlying the 

 uniformly hard, rocky, coral-bearing floor of the reef-flat is very slight (see 

 fig. 9) and there is remarkably little limestone sand cast ashore along the 

 entire southeast shore of the island, and as the reef-flat is not disturbed by 

 hurricanes and no strong currents wash over it, in this region at least, some of 

 the lime-stone sand due to disintegration of dead coral, etc., a[)parently dis- 

 appears in situ, being dissolved in the intestines of the large numbers of holo- 

 thurians and echini and fish which are constantly swallowing it; also even 

 weak currents of 40 feet per minute are effective in scouring and transporting 

 sand over reef-flats, as I found in Samoa in 1917. The crest of the litho- 

 thamnion ridge and those parts of the reef-flat to the seaward of it are not 

 disintegrated, for few echinoderms or other "sand feeders" can gain a foot- 

 hold upon it, on account of the heavy surge of the breakers in this region. 

 Thus the growth of nullipores and of the other veneering organisms is but 

 little interfered with in the region of the breakers, and this part of the reef- 



'Guppy, 1889, Scottish Geographical Magazine, vol. 5, p. 472. 

 'Wells, Journal Washington Academy of Science, vol. 5, p. 622, 1915. 



