128 CORAL-REEFS. 



supposed to have been formed on the new reef, and a ship 

 is anchored in the lagoon-channel. This section is in 

 every respect that of an encircling barrier-reef; it is, in 

 fact, a section taken 1 E. and W. through the highest point 

 of the encircled island of Bolabola ; of which a plan is 

 given in Plate II., Fig. i. The same section is more clearly 

 shown in the following woodcut (No. 5) by the unbroken 

 lines. The width of the reef, and its slope, both on the 

 outer and inner side, will have been determined by the 

 growing powers of the coral, under the conditions (for 

 instance, the force of the breakers and of the currents) 

 to which it has been exposed ; and the lagoon-channel 

 will be deeper or shallower, in proportion to the growth of 

 the delicately branched corals within the reef, and to the 

 accumulation of sediment, relatively, also, to the rate of 

 subsidence and the length of the intervening stationary 

 periods. 



It is evident in this section, that a line drawn perpen- 

 dicularly down from the outer edge of the new reef to the 

 foundation of solid rock, exceeds by as many feet as there 

 have been feet of subsidence, that small limit of depth at 

 which the effective polypifers can live, — the corals having 

 grown up, as the whole sank down, from a basis formed 

 of other corals and their consolidated fragments. Thus 

 the difficulty on this head, which before seemed so great, 

 disappears. 



As the space between the reef and the subsiding shore 

 continued to increase in breadth and depth, and as the 



1 The section has been made from the chart given in the Atlas of 

 the Voyage of the Coquille. The height of the island, according to 

 M. Lesson, is 4,026 feet. The deepest part of the lagoon-channel is 

 162 feet; its depth is exaggerated in the woodcut for the sake of 

 clearness. 



