226 J. D. Dana, Esq., on Coral Beefs and Islands. 



The mud-like deposits about coral reefs have been attri- 

 buted to the causes just mentioned, but without due consi- 

 deration. There is an unfailing and abundant 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 

 the coral island, and on the shores of the larger lagoons, 

 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 is a well-known fact, com- 

 mon on all shores exposed to the waves, coral or not coral, 

 and to this cause the sandy character is attributed. But 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 pul- 

 verized material remains as part of the shores. Thus the 

 fine material of the mud must be constantly forming on all 

 the shores, for the sands are perpetually wearing themselves 

 out ; but the mud accumulates only in the more quiet waters, 

 and within the lagoons and channels, where it settles, after 

 being washed out from the beaches. This corresponds ex- 

 actly with the facts ; and every lake, pool, or water of our 

 continents, illustrates the same point.* 



The coral world, as we thus perceive, is planted like the 

 land with a variety of shrubs and smaller plants, and the 



* 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 Holothurias ; and 

 other authors have thrown out the same suggestion. He cites as a fact, on the 

 authority of Mr Leisk, that certain fish browse on the living zoophytes ; and 

 from Mr Allan of Forres he learned also that Holothurias subsisted on them . 

 With regard to the facts here stated, I can make no definite assertion. Small 

 fish swarm about the branching clumps, and when disturbed, seek shelter at 

 once among the branches, where they are safe from pursuit : I have often wit- 

 nessed this fact, and never saw reason to suppose that they clustered about the 

 coral for any other purpose. It is an undoubted fact, however, as stated by 

 Mr Darwin, that fragments of coral and sand may be found in the stomachs of 

 these animals, though this is not sufficient evidence of their browsing on the 

 coral. The conclusion deduced by him from the facts may be justly doubted. 

 The fish and Holothurias, though numerous, are quite inadequate for the sup- 

 ply ; and moreover we have, as explained above, an abundant source of the finest 

 coral material without such aid. Motion of particle over particle, will necessarily 

 wear to dust, even though the particles be diamonds ; and this incessant grind- 

 ing action about reefs, accounts satisfactorily for the deposits of coral mud, 

 however great their extent. 



