^o . THE GREAT BARRIER REEF. 



reef— that kind of which the origin alone offers no difHciilty. Let the unbroken lines on the 

 diagram (No. i) represent a vertical section thiough the land and water, and the horizontal 

 (darkest) shading a section through the reef. Now as the island sinks down, either a few 

 feet at a time or quite insensibl}-, we may infer, from what we know of the conditions 

 favourable to the growth of coral, that the living masses bathed by the surf on the margin 

 of the reef will soon regain the surface. The water, however, will encroach little by little 

 on the shore, the island becoming lower and smaller, and the space between the edge of 

 the reef and the beach proportionately broader. A section of the reef and island in this state 

 after a subsidence of several hundred feet is given by the dotted lines. Coral islets are sup- 

 posed 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, and is, in fact, taken E. and W. 

 through the highest point of the encircled island ot Bolabola. The same section is more 

 clearly shown in the following woodcut (No. 2) 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 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 accu- 

 mulation 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 perpendicularly 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 polypipes 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 injurious effects of the sediment and fresh water borne down from the 

 land were consequently lessened, the greater number of the channels with which the reef in 

 its fringing state must have been breached, especially those which fronted the smaller streams, 

 will have become choked up by the growth of coral; on the windward side of the reef, where 

 the coral grows most vigorously, the breaches will probably have first been closed. In barrier 

 reefs, therefore, the breaches kept open by draining the tidal waters of the lagoon-channel, 

 will generally be placed on the leeward side, and they will still face the mouths of the 

 larger streams, although removed beyond the influence of their sediments and fresh water; 

 and this, it has been shown, is commonly the case. 



" Referring to the following diagram (No. 2 a), in which the newly-formed barrier reef is 

 represented by unbroken lines, instead of by dots, as in the former woodcut, let the work 



