44 CORAL-REEFS. 



the Maldiva atolls this is very conspicuous, and likewise in 

 some of the Caroline and Marshall Islands. In the former 

 large spaces consist of sand and soft day; and Kotzebue 

 speaks of clay having been found within one of the Marshall 

 atolls. No doubt this clay is calcareous mud, similar 

 to that at Keeling Island, and to that at Bermuda 

 already referred to, as un distinguishable from disintegrated 

 chalk, and which Lieut. Nelson says is called there pipe- 

 clay. 1 



Where the waves act with unequal force on the two sides 

 of an atoll, the islets appear to be first formed, and are 

 generally of greater continuity on the more exposed shore. 

 The islets, also, which are placed to leeward, are in most 

 parts of the Pacific liable to be occasionally swept entirely 

 away by gales, equalling hurricanes in violence, which blow 

 in an opposite direction to the ordinary trade-wind. The 

 absence of the islets on the leeward side of atolls, or when 

 present their lesser dimensions compared with those to 

 windward, is a comparatively unimportant fact; but in 

 several instances the reef itself on the leeward side, retain- 

 ing its usual defined outline, does not rise to the surface by 

 several fathoms. This is the case with the southern side of 

 Peros Banhos (Plate IV., Fig. 3) in the Chagos group, with 



1 I may here observe that on the coast of Brazil, where there is much 

 coral, the soundings near the land are described by Admiral Roussin, 

 in the Pilote du Brteil, as siliceous sand, mingled with much finely 

 comminuted particles of shells and coral. Further in the offing, for a 

 space of 1,300 miles along the coast, from the Abrolhos Islands to 

 Maranham, the bottom in many places is composed of "tuf blanc, 

 mele ou forme de madrepores broyes." This white substance, probably, 

 is analogous to that which occurs within the above-mentioned lagoons; 

 it is sometimes, according to Roussin, firm, and he compares it to 

 mortar. 



