66 B AETHER REEFS. Ch. II. 



(A A), and bear in mind that the plummet on the right 

 hand represents a depth of 1,200 feet, we must conclude 

 that the vertical thickness of these barrier coral-reefs is 

 very great. 



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- 

 building polypifers cannot flourish at great depths, — for 

 instance, it is highly improbable that they could exist 

 at above one-eighth of the depth represented by the 

 plummet on the right hand of the woodcut. Here then 

 is a great apparent difficulty — how were the basalt parts 

 of these barrier-reefs formed. It will perhaps occur to 

 some, that the actual reefs formed of coral are not of 

 great thickness, but that before their first growth the 

 sea had deeply eaten into the coasts of these encircled 

 islands, and had thus left a broad but shallow sub- 

 marine ledge, on the edges of which the corals grew ; 

 but if this had been the case, the shore would have 

 been invariably bounded by lofty cliffs, and not have 

 sloped down to the lagoon-channel, as it does in 

 many instances. On this view, 2 moreover, the cause 

 of the reef springing up at such a great distance from 



1 An East and West section across the Island of Bolabola and its 

 barrier-reefs is given in the fifth chapter, for the sake of illustrating 

 another point. The scale is -57 of an inch to a mile ; it is taken from 

 the Atlas of the Voyage of the Coquille, by Duperrey. The depth of 

 the lagoon-channel is exaggerated. 



2 The Eev. D. Tyerman and Mr. Bennett (Journal of Voyage and 

 Travels, vol. i. p. 215) have briefly suggested this explanation of the 

 origin of the encircling reefs of the Society Islands. 



