CORAL-REEFS. 



69 



***fcta^ 



South. 



Korth. 



1. Yanikoro, from the Atlas of the Voyage of the Astrolabe, by D. D'Urville. 



2. Gambier Island, from Beecbey. 



3. Maurua, from the Atlas of the Voyage of the Coquille, by Duperrey. 



The horizontal line is the level of the sea, from which on the right hand a 

 plummet descends, representing a depth of 200 fathoms, or 1,200 feet. The 

 vertical shading shows the section of the land, and the horizontal shading that 

 of the encircling barrier-reef : from the smallness of the scale, the lagoon- 

 channel could not be represented. 



AA. — Outer edge of the coral-reefs, where the sea breaks. 



BB.— The shore of the encircled islands. 



I must observe that if the sections had been taken in 

 any other direction across these islands, or across other 

 encircled islands, 1 the result would have been the same. 

 In the succeeding chapter it will be shown that reef-build- 

 ing polypifers cannot flourish at great depths, — for instance, 

 it is highly improbable that they can exist at a quarter of 

 the depth represented by the plummet on the right hand of 

 the woodcut. Here there is a great apparent difficulty — 

 how were the basal parts of these barrier-reefs formed ? It 

 will, perhaps, occur to some, that the actual reefs formed of 



1 In the fifth chapter an East and West section across the Island of 

 Bolabola and its barrier-reefs is given, for the sake 



of illustrating 



another point. The unbroken line in it (woodcut No. 5) is the section 

 referred to ; it is taken from the Atlas of the Voyage of the Coquille, by 

 Duperrey. The depth of the lagoon-channel is exaggerated. 



