Encirclingbarrier-reefsareofallsizes，fromthreemilestonolessthanforty-fourmilesindiameter；andthatwhichfrontsoneside，andencirclesbothends，ofNewCaledonia，is400mileslong。Eachreefincludesone，two，orseveralrockyislandsofvariousheights；andinoneinstance，evenasmanyastwelveseparateislands。Thereefrunsatagreaterorlessdistancefromtheincludedland；intheSocietyarchipelagogenerallyfromonetothreeorfourmiles；butatHogoleuthereefis20milesonthesouthernside，and14milesontheoppositeornorthernside，fromtheincludedislands。Thedepthwithinthelagoon-channelalsovariesmuch；
from10to30fathomsmaybetakenasanaverage；butatVanikorotherearespacesnolessthan56fathomsor363feetdeep。Internallythereefeitherslopesgentlyintothelagoon-channel，orendsinaperpendicularwallsometimesbetweentwoandthreehundredfeetunderwaterinheight：
externallythereefrises，likeanatoll，withextremeabruptnessoutoftheprofounddepthsoftheocean。
Whatcanbemoresingularthanthesestructures？Wesee[picture]
anisland，whichmaybecomparedtoacastlesituatedonthesummitofaloftysubmarinemountain，protectedbyagreatwallofcoral-rock，alwayssteepexternallyandsometimesinternally，withabroadlevelsummit，hereandtherebreachedbyanarrowgateway，throughwhichthelargestshipscanenterthewideanddeepencirclingmoat。