BARRIER REEFS. 
Cn. II. 
86 
(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-eightli of the depth represented by the 
plummet on the right hand of the woodcut. Here then 
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 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 Coquilla, by Duperrey. The 
depth of the lagoon-channel is exaggerated. 
2 The Rev. 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. 
