i 



On Coral Reefs and Isla^ids. 333 



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 o£ this kind of n}arerial 



m the self-triturating sands of the reefs acted upon by the moving 



^^aters. On the seaward side of the coral island, and on the shores 

 of the larger lagoons, where the surface rises into waves of much 

 niagnitude, 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 tfie sandy character is attributed. But in the 

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

 roughened for short intervalsj the trituration is of the gentlest 

 kind possible, and, moreover, the finely pulverized material re- 

 niains as part of the shores. Thus the fine material of the mud 

 niust 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 

 ^ater of our continents illustrates the same point.* 



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

 ^^'ith 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 

 JJiight 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 



^ , , H^,^ ™, _, ^ ^ 



Mr. Dar-R-in, in discussing the origin of tlie finer calcareous mud, (op. cit., p. 14,) 

 supposes that it is deriveJ, in part, from Fishes and Holotliurias, and other authors 

 «ave thrown out the same suggestion. He cites as a fact, on the authority of Mr. 

 A^iesk, that certain fi.h browse on the living zoophytes ; and from Mr. AUati. of For- 

 J"es, he learned also that Holothurias subsisted on them. With regtird to the facts 

 ?ere stated, I can make no definite assertion. Small fish swarm about the branch- 

 ing clump.^, and when disturbed, seek shelter at once among the branches, where 

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

 r ^"Ppose that they clu:itered about the coral for any other purpose. It is an un- 

 <ioubted fact, however, as stated by Mr. Darwin, tha"t fragments of coral and sand 

 ^y be found in the stomachs of these animals, thoui^h this is not sufficient evidence 

 ^ tlieir browsing on the coral. The conclusion deduced by him from the Axcts may 

 ^ justly doubted. The fish and Holothurias, though numerous, are quite inadi'quate 

 T ^^^ ^'^PP^y ; and, moreover, we have, as explained above, an abundant Fource of 

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

 cessarily wc-ar to dust, even though the particles be diamonds; and this mcessant 

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

 ever great their extent 



Second Series, YoL XII, Ko. 36.— N*ov., 1851. 43 



