rilFj Vr 1 • f ' /■• ( -OR A r ^ h-FFFS 



503 



channel. 'Ihis clianncl will be niMrc or less deep, acconlini; to 

 the rate of subsidence, to tlie amount of sediment accumulated 

 in it, and to tlie ,L;rd\\ th of the delicately branched corals which 

 can live there. The section in this state resembles in every 

 respect one drawn throui^h an (.'iicircled island : in fact, it is a 

 real section (on the scale of .517 of an inch to a mile; through 

 ]')olabola in the Pacific. We can now at once sec why encircling 

 barrier-reefs stand so far from the shoics which the)' front. 

 W'c can also perceive that a line drawn pcrpendicnlarl)- tlow n 

 from the outer edge of the new reef, to the foundation of solid 

 rock l-)encath the old fringing-reef will exceed by as nian\' feet 

 as there ha\e been feet of subsidence, that small limit of depth 



ith i-sltts on ir. Il'IV 



Tin 



-V.V. Outer edges of the li;irrier-reef at the level of the 

 i>f the incltiilej island. I C. 'rhe Irifioon-ch.Tiiiiel. 



.V"A", Outer edges of the reef, now converted into :m atoll, tj', The hcjoon of the new atoll. 



X.B. — According to the trtie scale, the depths uf the lagoon-channel and lagoon are much 

 exaggerated. 



SECTION OF COR.^L-kEEF. 



at which the effective corals can li\'e ; — the little architects 

 having built up their great walldikc mass, as the whole sank 

 down, upon a basis formed of other corals and their ccMisolidated 

 fragments. Thus the difficulty on this licad, which appeared 

 so great, disappears. 



If, instead of an island, we had taken the shore of a 

 continent fringed with reefs, and had imagined it ti"> h:i\c 

 subsided, a great straight barrier, like tliat of Australia or 

 X^ew Caledonia, separated from the land b\' a witle and deei) 

 channel, wcjuld evidently have been the resLilt. 



Let u."5 take our new encircling barrier-reef of which the 

 section is nf)W rcpresentctl b\' unbroken lines, aiitl which, as I 

 h.a\'C said, is a real section through liolabola, aiu1 let it ;.^ii 1 di 

 subsidiiiLT. i\s the barrier-reef slttwK- sinks down, the corals 



