FORMATION OF REEFS. 73 



The mud-like deposits about coral reefs have been attributed 

 to the causes just mentioned, but without due consideration. 

 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 

 common 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 pulverized material re- 

 mains 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 chan- 

 nels, where it settles, after being washed out from the beaches. 

 This corresponds exactly 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 elements 

 and natural decay are producing gradual accumulations of mate- 

 rial, like those of vegetation. The history of the growing reef 

 has consequently its counterpart among the ordinary occurrences 

 of the land about us. 



The progress of the coral formation is like its commencement. 

 The same causes continue with similar results, and the reader 

 might easily supply the details from the facts already presented. 

 The production of debris will necessarily continue to go on : a 

 part will be swept by the waves, across the patch of reef, into 



* Mr. Darwin, in discussing the origiu 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. 

 Liesk, that certain fish browse on the living zoophytes ; and from Mr. Allan, of For- 

 res, 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 branch- 

 ing clumps, and when disturbed, seek shelter at once among the branches, where 

 they are safe from pursuit : I have often witnessed this fact, and never saw reason 

 to suppose that they clustered about the coral for any other purpose. It is an un- 

 doubted 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 supply ; and, moreover, we have, as explained above, an abundant source of 

 the finest coral material without such aid. Motion of particle over particle, will ne- 

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

 grinding action about reefs, accounts satisfactorily for the deposits of coral mud. how- 

 ever great their extent. 



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