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192 



GEOLOGY 



9 



[Sect YL 



Fringing reefs. Whenever it is practicable, soundinrrg 

 ought to be taken at short ascertained distances, from 

 close to the breakers in a straight line out to sea, so tlr * 



a sectional outline might be protracted on paper. In 

 those cases in which the bottom descends by a set of 

 ledges or steps, their form ought to be particularly 

 attended to ; and whether they are covered with sand or 

 by dead or living coral ; and whether the corals dilll., on 

 the different ledges : the same points should be attended 

 to within the lagoon, wherever its bed or shore is step- 

 formed : the origin of these steps or ledges is at present 

 obscure. In the Indian and Pacific oceans there are 

 entire reefs, having the outline of atolls or lagoon-islands 

 lying several fathoms submerged ; there are likewise de- 

 fined portions of reefs both in atolls and in encircling reefs 

 similarly submerged. It would be particularly desirable 

 to ascertain what is the nature of these submerged sur- 

 faces, whether formed of sand or rock or living or dead 

 corals. In some cases two or more atolls are united 

 by a linear reef; the form of the bottom on each side 

 of this connecting line ought to be examined. Where 



tolls or reef-encircled islands stand very near each 

 other, the depth between them might be attempted by 

 deep soundings: the bottom has been struck between 

 ome of the Maldiva atolls. Generallv the form and 



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nature of the reefs encircling islands ought to he com 



pared 

 atolls. 



Ill 



every respect with the annular reefs forming 



On the shores of every kind of reef, especially of atolls 

 and of land encircled by barrier reefs, evidence of the 

 slow sinking of the land should be particularly sought for; 





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