954 FORMATION OP BARRIER REEFS IN BOUGAINVILLE STRAITS, 



On the other hand, should the submarine slope have a more 

 rapid descent, the lower limit of the belt of detritus may extend 

 far beyond the depths in which reef-corals can thrive : in such a 

 case no barrier-reef will form, and the original fringing-reef will 

 continue to grow outwards on its own talus. On this view the 

 occurrence of barrier-reefs and of fringing-reefs on different parts 

 of the coast of the same island may be readily explained as due to 

 the different degrees of inclination of the submarine slope. 



Keeping in view the foregoing explanation of the formation of 

 a barrier-reef in a district which may for a long period have exper- 

 ienced no change in the relative positions of land and sea, we can 

 perceive how in an area of elevation line after line of barrier-reef 

 will be formed as from time to time fi-esh portions of the sea-bottom, 

 previously below the reef-coral zone, are brought up within the 

 depths in which reefs commence their growth ; line upon line of 

 barrier-reef will be thus advanced, each growing up along the lower 

 limit of the belt of detritus derived from the line of reef inside 

 it. In process of time the elevating movement assisted by the 

 accumulation of sediment, the growth of branching corals, and the 

 reclaiming agency of the mangrove, will bring about the filling up 

 of the passages or lagoon-channels between the lines of reef, until 

 at length a tract of land is produced rising gradually from the sea- 

 border to the interior but with the ancient lines of barrier-reef 

 still indicated by ridges of coral-limestone on its surface. Such in 

 fact is in my mind the history of the formation of the Shortlaud 

 Islands and I opine of the western extremity of the Choiseul Island. 

 In the former locality we have the original Island of volcanic 

 formation in the North-west corner, from which, as from a nucleus, 

 line after line of barrier-reef has been advanced in a south-easterly 

 direction, forming ultimately, during thecontinuance of the elevation, 

 the large Island of " Alu." Should this elevating movement beat 

 present suspended, as would appear to be indicated by the great 

 width of the reef-flats still over-flowed by the sea on the weather 

 coasts of the outlying islands, there yet remains a considerable 

 addition to be made to the sea-border of " Alu " by the filling up 

 of the passages between the lines of islands which represent 



