132 THEORY OF THE FORMATION Ch. V. 



taken E. and W. through the highest point of the 

 encircled island of Bolabola, 1 of which a plan is given 

 in Plate I. fig. 5. The same section is more clearly 

 shown in the following woodcut (No. 6) by the un- 

 broken 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 

 different conditions, for instance, of the force of the 

 breakers and 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 per- 

 pendicularly 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 con- 

 solidated 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 injurious effects of the sediment and fresh water 



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

 Voyage of the Coquille. The scale is '57 of an inch to a mile. 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. 



