36 ATOLLS. Ch. I. 



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 Ber- 

 muda already referred to, as undistinguishable 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 length on the more 

 exposed shore. The islets, also, which are placed 

 to leeward as regards the trade-wind, are in most 

 parts of the Pacific liable to be occasionally swept 

 entirely away by gales, equalling hurricanes in vio- 

 lence, which blow in the opposite direction. The 

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

 when present, their lesser dimensions compared with 

 those to windward, is a comparatively unimportant 

 fact ; but it is remarkable that in several instances 

 the reef itself, although retaining its usual defined 

 outline, does not rise to the surface by several fathoms 

 on the leeward side. This is the case with the 

 southern side of Peros Banhos (Plate I. fig. 9) in the 



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

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

 Eoussin. in the Pilote du Bresil, 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, 

 niele 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 Eoussin, firm, and he compares it to 

 mortar. 



