228 CORALS AND CORAL L8LAND8. 



source of this kind of material in the self -triturating sands of 

 the reefs acted upon by the moving waters. On the seaward 

 side of coral islands, and on the shores of the larger la- 

 goons, where the surface rises into waves of much magnitude, 

 the finer portions are carried off, and the coarser sand remains 

 alone to form the beaches. This making of coral sand and 

 mud is just like that of any other kind of sand or mud. It 

 takes place on all shores exposed to the waves, coral or not 

 coral, and in every case the gentler the prevailing movement 

 of the water, the finer the material on the shore. In the 

 smaller lagoons, where the water is only rippled by the winds, 

 or roughened for short intervals, the trituration is of the 

 gentlest kind possible, and, moreover, the finely pulverized 

 material remains as part of the shores. Thus the fine mater- 

 ial of the mud must be constantly forming on all the shores, 

 for the sands are perpetually wearing themselves out ; but the 

 particles of the fine mud, which is washed out from the beach 

 sands, accumulates only in the more quiet waters some dis- 

 tance outside of the reef, and within the lagoons and channels, 

 where it settles. This corresponds exactly with the facts ; and 

 every small lake or region of quiet waters over our continent, 

 illustrates the same point. 



Mr. Darwin, in discussing the origin of the finer calcareous 

 mud, (op. cit., p. 14), supposes that it is derived in part from 

 fishes and Holothurians ; and other authors have thrown out 

 the same suggestion. He cites as a fact, on the authority of 

 Mr. Liesk, that certain fish browse on the living zoophytes ; 

 and from Mr. Allan, of Forres, he learned also that Holothu- 

 rians subsisted on them. With regard to the facts here stated, 

 can be made no positive assertion. Small fish swarm about the 

 branching clumps, and when disturbed, seek shelter at once 

 among the branches, where they are safe from pursuit. The 



