886 Proceedings of the Boyal Society 
Keeping in view the foregoing explanation of the formation of 
a barrier-reef in a district which may for a long period have experi- 
enced no change in the relative positions of land and sea, we can 
perceive how in an area of elevation line after line of harrier-reef 
will he formed as from time to time fresh portions of the sea-hottom, 
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 Shortland 
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 the continuance of the eleva- 
tion, the large island of Alu. Should this elevating movement be 
at present suspended, as would appear to be indicated by the great 
width of the reef -flats still overflowed 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 elevated 
barrier-reefs on its weather coasts. Such a process is in actual 
operation at the present time in the passages, the encroachment 
of the mangrove on either side and the upward growth of coral 
in the channels being the agencies at present effecting this operation. 
These remarks may be made more clear by a reference to the 
sections of the Shortland Islands, which are given in my paper 
on the recent calcareous formations {Trans. Edin. Roy. Soc., vol. 
xxxii. part iii.), and in vol. ix. part 4, of the Proceedings of the 
Linnean Society , JV.S. W. 
It follows from this view of the formation of barrier-reefs that 
