260 Mr. John Murray [March 16, 



As has been stated, the lagoon in many of the smallest atolls has 

 been filled up, but this never ajDpears to happen in atolls with a 

 diameter of over two miles unless there be distinct evidence of 

 ui)heaval. In perfectly formed atolls, that is, those in which the 

 reefs are nearly continuous throughout, the deepest water is found 

 towards the centre of the lagoon, and there is a relation between this 

 depth and the depth of water be3^ond the outside reefs. In North 

 and South Minerva reefs, in the South Pacific, where the outside 

 depths are very great, there are depths down to 17 fathoms in the 

 lagoons, which are apparently clear of coral heads. Here we may 

 suppose that the central parts of the lagoon have for a long time been 

 exposed to the solvent action of sea-water, owing to the slow lateral 

 growth of the reef as a whole. In the same regions the Elizabeth 

 and Middleton reefs, which are about the same size, have only four 

 or five fathoms within the lagoons, and the depths outside the reefs 

 are at the distance of a mile mostly within the 100 fathom line, and 

 sometimes less than 50 fathoms. There are also many coral heads 

 within the lagoons. Here we may suppose the atolls to be more 

 recent and to have extended more rapidly than in the case of the 

 Minerva reefs. If the depths beyond the reefs be taken into con- 

 sideration, then there is usually a direct relation between the depth 

 of the lagoon and its diameter. The greatest depths, even in the 

 largest atolls, do not exceed 50, or at most 60, fathoms ; they are 

 usually much less. In atolls which are deeply submerged, or have 

 not yet reached the surface, which have wide and dcej) ojDcnings into 

 the lagoon-like spaces, this relation may not exist. In these instances 

 the secretion and deposition of carbonate of lime may be in excess of 

 solution in all parts of the lagoon. It is only when the atoll reaches 

 the surface, becomes more perfect, and its lagoon waters consequently 

 less favourable to growth, that the solution of the dead corals and 

 calcareous debris exceeds any secretion and deposition that may take 

 place throughout the whole extent of the lagoon ; it is then widened 

 and deepened, and formed into a more or less perfect cup-like 

 depression. 



The whole of a coral reef is permeated with sea-water like a 

 sponge ; as this water is but slowly changed in the interior parts it 

 becomes saturated, and a deposition of crystalline carbonate of lime 

 frequently takes place among the interstices of the corals and coral 

 debris. In consequence of the solution of coral debris and the re- 

 deposited lime occupying less space, large cavities are formed, and 

 this process often results in local depression in some islands, as, for 

 instance, in Bermuda. At many points on a reef where evaporation 

 takes place there is a deposition of amorphous carbonate of lime 

 cementing the whole reef materials into a compact conglomerate-like 

 rock. 



The fragments of the various organisms broken off from the outer 

 edge during gales or storms are piled up on the upper siu'face of the 

 reef, and eventually ground into sand, the result being the formation 



